Anatomy & Physiology for Midwives 3: Third Edition

Chapter 9. Embryo development and fetal growth

Learning objectives

• To describe the formation of the bilaminar and trilaminar embryonic discs in weeks 2 and 3 of development.

• To outline the events involved in folding of the embryonic disc into the characteristic shape of the human embryo.

• To define key embryological terms: gastrulation, neurulation, primitive streak, somites and notochord.

• To outline events in development in the first 8 weeks.

• To describe developmental characteristics of the fetal organ systems.

• To discuss factors affecting fetal growth and the implications these have for future health.

• To relate the timing of development with sensitive periods and to appreciate the developmental factors limiting survival of a preterm baby.

• To briefly outline how common fetal abnormalities are related to abnormal embryonic development.

Introduction

During the 9 months of pregnancy, the single cell of the zygote divides to produce 6 billion cells of the mature fetus. On average, an adult cell is the product of about 47 cell divisions from the zygote, at least 40 of which occur before birth. The first 3 weeks of development are often described as the pre-embryonic period when the cells differentiate into germ layers from which all organs and tissues develop. The sequence of events in this stage, albeit with a different time course, is similar in all sorts of animals, including Drosophila (fruit fly), nematodes, amphibians and birds as well as mammals. The embryonic stage, embryogenesis or organogenesis, lasts from week 4 to week 8 in the human. During this time, the organ systems are established and the embryo develops distinct human characteristics. The fetal stage, from week 9 to birth, is largely a period of growth, during which time the systems become more refined and mature and the fetus gains weight, ready to function at birth. Fetal age is timed from fertilization, whereas pregnancy is dated from the first day of the last normal menstrual period. This means that the timing of the pregnancy is 2 weeks more than the true fetal age. The average length of pregnancy is 280 days (40 weeks) when the fetus is 266 days old (38 weeks). Understanding the key concepts of embryology and fetal development is important in monitoring the well-being of the fetus and developing appropriate healthcare programmes to promote better reproductive health outcomes. This field has important applications in prenatal diagnosis and treatments as well as prevention and management of infertility and birth defects. Pregnant women are also obviously interested in knowing how their baby is changing during the duration of the pregnancy. (Development in the first week after fertilization is described in Chapter 6.)

Chapter case study

Zara is 5 feet and 2 in. tall (155 cm) and, prior to pregnancy, wore size 8 clothes; she wears size 3 (European size 35.5) shoes. Her husband, James, is 6 feet tall (180 cm) and has quite a broad, muscular frame and has size 11 feet (European size 46). Zara has frequently discussed with her midwife her concerns that she will have a big baby and problems in labour because James is tall and has a large frame.

• How can the midwife reassure Zara that the baby's growth is unlikely to be disproportionate to her size and what physiological measurements and observations can be made to support this?

• If Zara was found to be carrying a large baby how would the midwife be able to recognize this and what could be the possible explanations for this?

• How would having a large baby affect the plan of care for Zara; what are the potential concerns and how could they be mediated?

Week 2

By the end of the first week, the blastocyst has entered the uterine cavity, hatched out of the zona pellucida and started the process of implantation into the endometrial wall. Two types of cells are evident: the outer trophoblast (see Chapter 6) and the inner cell mass (or embryoblast). The inner cell mass gives rise to tissues of the embryo and also contributes towards some of the extraembryonic membranes. The undifferentiated cells from the blastocyst are called stem cells and have some potential therapeutic uses (see Box 9.1). At about day 7 after fertilization, the cells of the inner cell mass start to proliferate and differentiate rapidly. The inner cell mass becomes flattened into a bilaminar embryonic disc with the cells forming two distinct layers (Fig. 9.1). The cells adjacent to the blastocyst cavity appear distinctly cuboidal. These form the hypoblast or primary endoderm layer, which gives rise to the future gut and its derivatives. The upper layer of cells is formed of columnar epiblast cells, which will differentiate into the ectodermal layer. Some of the epiblast cells spread laterally to form the amnioblasts of the amniotic membrane that encloses the amniotic cavity. Cells from the hypoblast layer migrate forming a membrane called the exocoelomic membrane or Heuser's membrane which lines the cytotrophoblast so the blastocyst cavity is also enclosed. This is the cavity that will become the primitive (primary) yolk sac. There are two waves of endoderm cell remodelling of the blastocyst cavity, which form initially the primary yolk sac and then, at the beginning of the fifth week, the definitive (secondary) yolk sac (Fig. 9.2). The formation of the definitive yolk sac creates the chorionic cavity and the extraembryonic mesoderm, which gives rise to the vascular structures of the placenta (see Chapter 8). The definitive yolk sac synthesizes several proteins including alpha-fetoprotein (AFP). The bi-laminar disc lies between two fluid-filled cavities: the amniotic cavity on the epiblast (ectoderm) side and the yolk sac cavity on the hypoblast (endoderm) side. (The primitive yolk sac will shrink away from the cytotrophoblast in the fourth week, creating a new cavity called the chorionic cavity or extraembryonic coelom, which fills with fluid and becomes the largest cavity in the developing conceptus.)

Box 9.1

Stem cells, totipotency and pluripotency

Stem cells are unspecialized cells that have the ability to self-renew and to differentiate into a number of cell types. This ability to produce a variety of differentiated cell types means that stem cells are a potential source of replacement cells that could be used to treat variety of diseases and repair damaged tissues. There are several categories of stem cells defined according to the variety of cells they are able to produce. Totipotent stem cells are able to produce all of the cells present in the fetus and adult together with those in the extraembryonic tissues such as placenta. Pluripotent cells are able to produce any cell in the body, multipotent cells can produce particular types of related cells, for example blood cells, whereas oligopotent (a few types of very similar cells) and unipotent (one cell type) have progressively more restricted lineages.

The zygote and cells from only the first few divisions are termed ‘totipotent’. At the fourth cleavage division after fertilization, when the 8-cell embryo divided to form 16 cells, totipotency is lost. Genetic screening can be carried out at the 8-cell stage by the technique of pre-implantation genetic diagnosis by removing one of the eight cells for analysis. Since all cells at this stage are totipotent, the zygote will develop normally from the seven remaining cells. Also, if the cells are separated at this stage and allowed to implant, identical offspring will develop; this is a type of cloning. Following the loss of totipotency, the embryonic cells differentiate into pluripotent stem cells. These pluripotent cells can form any of the three types of germ cell layer (endoderm, mesoderm or ectoderm) and are therefore capable of becoming any cell in the body but cannot form a unique individual because they cannot form extraembryonic mesoderm and placental tissue.

There are a number of sources of stem cells which are usually classified as being embryonic (pluripotent) or adult (multipotent) stem cells. Embryonic stem cells (ESC) can be harvested from inner cell mass of the blastocyst (the trilaminar embryonic disc). Research using ESC is controversial because human ESC are taken from early embryos which are destroyed. ESC can be obtained from a cloned embryo which would make them genetically compatible with the recipient and therefore avoid the issue of immune rejection. A potentially important and recent advance is to ‘persuade’ differentiated cells to return to the undifferentiated stem cell state by expression of specific genes, a technique referred to as induced pluripotency (iPS). This also offers the advantage that somatic cells from an adult can be converted into stem cells and then be used to treat diseases in the same person, again, avoiding the problem of tissue immune rejection.

Adult stem cells are derived from progenitor cell populations such as bone marrow cells which can be differentiated into liver, kidney, muscle and nerve cells as well as blood cells. Skin dermis cells can be transdifferentiated into many types of tissue including neurons, smooth muscle cells and fat cells. It is also possible that brain cells could be harvested from dead organ donors. A major concern of stem cell use is to control stem cell division to ensure that tissues are regenerated but tumours do not grow.

Most adult stem cells are multipotent but some stem cells from the umbilical cord and cord blood cells are pluripotent. It is possible to reprogram multipotent adult stem cells to become pluripotent. In some countries, parents are asked whether they would like to have their baby's cord blood cells frozen in case they can be used later in life to treat a disease. The blood is taken from the umbilical vein of the cleaned cord once the baby has been delivered; it is checked for infectious agents, tissue-typed and then stored in liquid nitrogen. Stem cells can be used either autologously (for the person they came from) or as an allogenic treatment, where the donor and recipient are different individuals. The information given to new parents suggests a much wider use of the stem cells harvested from their baby's umbilical blood but many of these applications are currently at the research stage. To date, stem cells have been used in humans to treat leukaemia and restore eyesight using limbal stem cell therapy. The treatment of type-I diabetes, spinal cord injuries, neurodegenerative and other diseases is an active area of research.

Fig. 9.1 Differentiation of the inner cell mass into the bilaminar disc: (A) 7 days; (B) 8 days; (C) 9 days. (Reproduced with permission from Fitzgerald and Fitzgerald, 1994.)

Fig. 9.2 First-trimester gestational sac about 3 weeks postfertilization, showing chorionic plate (CP) surrounding entire sac, extraembryonic coelom (ECC), amniotic cavity (AC) and embryo (E) with its secondary yolk sac (SYS) providing nutrients. (Reproduced with permission from Jauniaux et al., 2003.)

At the end of the second week, a region of endodermal cells starts to thicken and become columnar, forming the prochordal plate (Fig. 9.3). This marks the cranial region (head end) and is the site of the future mouth. The prochordal plate is also important in influencing further development of the cranial region (Box 9.2).

Fig. 9.3 Formation of the prochordal plate (future mouth) and primitive streak on the bilaminar and trilaminar embryonic discs.

Box 9.2

Totipotency and pluripotency

The zygote and cells of the early blastocyst are termed ‘totipotent’ which means that each cell has the ability to develop into all cells of the organism and form all the types of body tissue. At the fourth division after fertilization, when the eight cells present divide forming 16 cells, totipotency is lost. Genetic screening can be carried out by pre-implantation genetic diagnosis before the fourth division by removing one of the eight cells for analysis; the zygote will develop normally from the seven remaining cells. If the cells are separated and implanted, identical offspring will develop; this is a type of cloning. Following the loss of totipotency, the embryonic cells differentiate into pluripotent stem cells. These pluripotent cells can form any of the three types of germ cell layer: endoderm, mesoderm or ectoderm but cannot form a unique individual because they cannot form extraembryonic mesoderm and placental tissue.

By the second week, according to the embryologist's ‘rule of twos’, the following have taken place:

• two germ layers have formed: the endoderm and the ectoderm

• two trophoblastic layers have formed: cytotrophoblast and syncytiotrophoblast

• two waves of remodelling have occurred: that of the blastocyst into the primary and then the definitive yolk sac

• two novel cavities have formed: the amniotic cavity and the chorionic cavity

• two layers have formed from the extraembryonic mesoderm.

Week 3

At this stage, when the woman may first realize she is pregnant, embryo development is rapid. A line of epiblast cells, starting from the caudal region (tail end) at the other side from the prochordal plate, undergoes very rapid cell division, forming the primitive streak in the midline (see Fig. 9.3). The cells of the primitive streak form a groove and then invaginate (move inwards) to spread between the epiblast and hypoblast layers. The bilaminar disc is therefore converted into a trilaminar disc consisting of three germ layers (ectoderm, mesoderm and endoderm), which give rise to specific tissues of the body (Fig. 9.4). The middle layer is the mesoderm, from which connective tissue, smooth muscle, the cardiovascular system and blood, the skeleton and the reproductive and endocrine systems develop (Fig. 9.5). The epiblast becomes the ectoderm, which will develop into the epidermis, central and peripheral nervous systems and the retina. Therefore, the ectoderm, which will give rise to the skin, is in contact with the amniotic cavity from very early on in embryonic development. The hypoblast becomes the endoderm, from which epithelial linings and some glandular structures will form. The three germ layers interact, generating signals that induce cellular interactions and cause structural alterations and more complex interactions.

Fig. 9.4 The invagination of cells of the primitive streak between the ectodermal and endodermal layers creates a trilaminar embryonic disc. (Reproduced with permission from Fitzgerald and Fitzgerald, 1994.)

Fig. 9.5 The neural and surface ectoderm, the endoderm and the mesoderm will differentiate into future tissues of the body.

The endodermal prochordal plate is fused to the ectoderm forming the oropharyngeal membrane (future mouth). Below the primitive streak, there is another area of fusion between the ectoderm and endoderm; this is the cloacal membrane (the future anus). Rare birth complications such as imperforate anus may arise from abnormal development of the cloacal membrane. Some mesoderm cells migrate towards the prochordal plate forming a cord of adhesive cells (Fig. 9.6). This is the notochordal process, which develops a lumen forming the notochord canal. The notochord evolves into a cellular rod-like tube, which gives the trilaminar disc a degree of rigidity and defines the central head–tail axis of the embryo. If identical twins are going to develop, there are two parallel notochords. If there are two notochords that cross, conjoined (Siamese) twins will result; on the position where the notochords cross dictates where the twins will be conjoined, for example twins with cephalic joining have a higher cross-over point of their notochords than twins who are joined at the hips (Spitz, 2005). The notochord establishes the development of the axial skeleton (bones of head and spinal cord) and the neural plate, which gives rise to the primitive nervous system. The vertebral column forms around the notochord and the notochord induces neurulation, the formation of the neural tube and early nervous system (see below). During the third week, aggregates of mesoderm on either side of the notochord form pairs of bead-like blocks called somites, which direct the segmented structure of the body and induce the overlying ectoderm to form structures of the nervous system.

Fig. 9.6 Notochord formation: (A) 17 days and (B) 18 days. (Reproduced with permission from Goodwin, 1997.)

The formation of the primitive streak, the three germ layers, the prochordal plate and the notochord is described as gastrulation. Gastrulation marks the beginning of morphogenesis, the emergence and development of body form and structure. It begins with the appearance of the primitive streak at day 14. The primitive streak defines the time when experimental manipulation of human embryos is legally obliged to stop under the terms of the UK Human Fertilization and Embryology Act of 1990 (amended 2008). Gastrulation is a very sensitive stage of embryogenesis; the cell populations are very vulnerable to teratogenic insult at the beginning of the third week of development (Sadler, 2010). For instance, high levels of alcohol can kill cells in the craniofacial region of the embryonic disc affecting brain and face development. The very rare tumours of the neonate can be a result of remnants of primitive streak proliferating to form sacrococcygeal tumours. During the third week of development, as well as gastrulation, the primitive nervous system and cardiovascular system begin to develop.

Box 9.3 is a summary of the events taking place in weeks 1–3.

Box 9.3

Summary of pre-embryonic period: weeks 1–3

Week 1: fertilization to produce zygote

• Cleavage of zygote while travelling in uterine tube

• Cell division without increase in mass to form morula

• Fluid accumulation: hollow blastocyst formed

• ‘Hatching’ out of zona pellucida

• Blastocyst cells differentiate into trophoblast and inner cell mass

• Implantation in decidual wall

Week 2: inner cell mass forms bilaminar embryonic disc of hypoblast and epiblast

• Trophoblast differentiates into dividing cytotrophoblast and invasive syncytiotrophoblast (see Chapter 8)

• Lateral movement of cells from epiblast layer encloses yolk sac, forming the extraembryonic mesoderm

• Prochordal plate (mouth) develops at caudal end

• Day 14: primitive streak develops

Week 3: gastrulation

• Cells from primitive streak invaginate and migrate between the epiblast and the hypoblast forming the mesoderm

• Trilaminar disc of three germ layers: ectoderm (epiblast), mesoderm and ectoderm (hypoblast)

• Notochord forms, inducing development of the neural plate and giving rise to axis of development

• Somites become evident

• Neurulation begins

Weeks 4–8: organogenesis

During this period of embryonic development, the trilaminar disc folds into a C-shaped cylindrical embryo and all the major structures and organ systems are established. However, apart from the cardiovascular system, few of the systems function. Organogenesis, the development of the organ systems, is a critical period during which the processes are susceptible to external influences that can cause disruption and subsequent serious congenital abnormalities. By the end of the eighth week, the embryo becomes known as the fetus and has a distinct human appearance (Fig. 9.7). Human development can be crudely classified as three types:

• growth: cell division

• morphogenesis: development of form, which involves movement of sheets and masses of cells

• differentiation: maturation of cells forming tissues and organs capable of specialized function.

Fig. 9.7 (A) 4-week-old and (B) 8-week-old fetus. ((A) Reproduced with permission from Fitzgerald and Fitzgerald, 1994.)

Growth is achieved by hyperplasia (cell division) and hypertrophy (increase in cell size). Initially the cells are stem cells which are similar and not differentiated or specialized into any particular cell type. They differentiate into 1 of the 350 different types of cell found in the body in two phases. Before differentiation occurs, there is a stage of determination during which the cell becomes restricted in its capability to develop along different pathways. As the cells differentiate fully, they develop specific morphological and functional characteristics.

Differentiation is often orchestrated by the establishment of a signalling centre or polarizing region in a small bud of undifferentiated cells, as occurs in the development of the vertebrate limb. This process in which cells in one place influence the surrounding cells to develop in a specific way is known as induction. Induction involves the surrounding cells (‘inducers’) to produce cellular signals which have an effect on the responding cells (‘responders’) via cell receptors. Competence is the capacity to respond to the signal from an inducer which requires the responding tissue to be activated by a competence factor (Sadler, 2010). These interactions often involve epithelial cells, which are usually joined together forming tubes or sheets, and mesenchyme cells which are more dispersed. For instance, the epithelial cells forming the lining of the gut interact with the neighbouring mesenchyme cells to form the gut-associated organs such as the pancreas and liver and the epithelial tissue interacts with the limb mesenchyme cells to produce the initial limb buds. Continued signalling or ‘crosstalk’ between the different cell types allows differentiation to progress. The crosstalk occurs by the cells producing growth and differentiation factors which act at a paracrine (involving diffusible factors) or juxtacrine (involving non-diffusible factors) level.

Paracrine factors act by triggering a signalling transduction pathway in which a signalling molecule (or ‘ligand’) interacts with its receptor often conferring enzymatic activity to the receptor so it is able to initiate a cascade of protein phosphorylation changes that terminate with a transcription factor being activated. The transcription factor can then activate or inhibit gene expression. Juxtacrine factor signalling involves proteins on the surface of one cell or in the extracellular matrix interacting with the receptor on the surface of another cell or signals being transmitted from one cell to another via gap junctions. Differentiation can allow some plasticity. Branching morphogenesis is the formation of branched epithelial tubules which is essential to the development of several tissues including the kidneys, lungs, breasts and salivary glands.

One of the cornerstones of embryology is the concept that germ cells of the three layers of the developing embryo migrate to the final destinations (Horwitz and Webb, 2003). The migrating cells are thought to be polarized (have a front and back), to sense chemical signals and ‘home’ towards them by amoeboid movement. In gastrulation, cell migration leads to the formation of the three layers of the trilaminar disc which then migrate to targets where they differentiate. The muscle precursor cells migrate from the somites to their targets in the limbs. Failure of cells to migrate at all or to the correct location is thought to result in abnormalities or to have life-threatening consequences; for instance, congenital defects in brain development leading to mental disorders are ascribed to defects in neuronal migration. Cell migration also occurs in post-natal life and is central to homeostasis such as effective immune responses and repair of injured tissues in the wound healing process. Migration can also occur in pathological processes, including vascular disease and tumour metastasis, where some tumour cells migrate to new sites where they form secondary tumours. However, recent research has questioned whether germ cells do actually migrate at all during ontogeny (Freeman, 2003).

Apoptosis or programmed cell death is another mechanism important in embryonic development. Apoptosis involves the cells effectively autodestructing in a precisely timed manner. Embryogenesis also involves cell recognition and adhesion.

Folding

The disc-like arrangement of the germ layers is converted into a recognizable vertebral embryo by folding in the fourth week of development. Folding is due to a differential rate in growth of the different parts of the embryo. The embryo is in a contained space so as it grows, it curves and ridges of tissues form. The embryonic disc grows rapidly particularly in length, because of the growth of the brain and tail, so it has to fold. Although this is a momentous stage of development, relatively little is known about it. The yolk sac does not grow and, as the outer rim of the endoderm is attached to the yolk sac, the embryo becomes convex. Folding occurs at the cephalic (head) and lateral regions on day 22 and at the caudal (tail) end of the embryo on day 23 (Fig. 9.8). The cephalic, lateral and caudal edges of the embryonic disc are brought into apposition and the layers fuse along the midline, which converts the endoderm into the gut tube. Initially the foregut and the hindgut fuse, leaving the midgut open to the yolk sac. The folds cause a constriction between the embryo and yolk sac. The yolk sac gives rise to the primitive gut. The amnion expands, enveloping the connecting stalk and neck of the yolk sac, forming the umbilical cord. Folding is precisely coordinated and is controlled. Failure at this point results in conditions such as omphalos and gastroschisis. The developmental pattern involves synchronized tissue communication and interaction. Adjacent tissues induce changes in the movement and behaviour of neighbouring cells. Signals integrating genetic and environmental influences control cell proliferation, migration and apoptosis (Bard and Weddon, 1996). These signals, which may be diffusible molecules or direct physical contact, direct the expression of particular genes in the responding cells. Although all cells have the same DNA in their nuclei, depending on the signal received, some will express certain genes but not others. So, for instance, a skin cell expresses the genes that control the behaviour of a skin cell because they are switched on by the signals skin cells receive. A liver cell has the same genes as the skin cell but expresses different genes.

Fig. 9.8 Folding of the embryonic disc into the fetal morphology: (A) 21 days; (B) 22 days; (C) 23 days; (D) 25 days. (Reproduced with permission from Goodwin, 1997.)

The organization of the basic body plan

Techniques and concepts used to study molecular genetics (how the genetic code is expressed) in bacteria and Drosophila can be applied to mammalian embryogenesis, including human development. The DNA in the nucleus sets up a basic body plan, which establishes the pattern of the early embryo. The genes that control the basic body plan are the same in very diverse species. A highly conserved region of about 180 base pairs of DNA, known as the homeobox, is found in the genes that regulate the craniocaudal (head–tail) axis of embryonic development in almost all species studied (Murtha et al., 1991). The homeobox encodes a protein domain called the homeodomain which can bind DNA specifically. Homeobox genes encode transcription factors which switch on cascades of other genes, for instance all the ones needed to make a particular body part. Other morphogenic agents, signals and growth factors activate the homeobox genes. Hox genes are a particular subgroup of homeobox genes which are found in a special gene cluster, the HOX cluster. Hox genes determine the patterning of the body axis. They direct the identity of particular body regions, determining where limbs and other body segments will develop in the fetus. Limb abnormalities such as polydactyly may result from abnormal Hox genes. There appears to be a series of three sequential steps in the conversion of the oval trilaminar embryonic disc into the cylindrical configuration with the endoderm on the inside, the ectoderm on the outside and the mesoderm in between (Fig. 9.9) (Carlson, 2008). These steps result in segmentation of the embryo. Gap genes subdivide the embryo into broad regional domains. Pair-rule genes are involved in the formation of individual body segments and segment-polarity genes control the anterior–posterior organization of each segment (De Robertis et al., 1990). As the embryo develops, the segmental plan becomes less evident; remnants can be seen in the arrangement of the backbone and ribs and in the organization of the spinal nerves.

Fig. 9.9 Organization of the vertebrate body plan: three steps in the conversion.

Box 9.4 summarizes the events taking place during weeks 4–8.

Box 9.4

Summary of embryonic period: weeks 4–8

4th week

• Neural tube fusing but neuropores open at rostral (anterior) and caudal ends

• Folding produces characteristic C-shaped curved embryo

• Otic pits present (primitive ear)

• Optic vesicles formed

• Upper limb buds appear, then lower limb buds

• Three pairs of brachial arches present

• Beating heart prominent

• Forebrain prominent

• Attenuated tail

• Rudiments of organ systems established

• Rostral neuropore, then caudal neuropore, close

• CRL 4–6 mm

5th week

• Rapid brain development and head enlargement (cephalization)

• Facial prominences develop

• Upper limb buds become paddle-shaped

• Lower limb buds are flipper-like

• Mesonephric ridges denote position of mesonephric (interim) kidneys

• CRL 7–9 mm

6th week

• Joints of upper limbs differentiate

• Digital rays (fingers) of upper limbs evident

• External ear canal and auricle (pinna) formed

• Retinal pigment formed so eye is obvious

• Head very large, projects over heart prominence

• Reflex responses to touch

• CRL 11–14 mm

7th week

• Notches between digital rays partially separate future fingers

• Liver prominent

• Rapidly growing intestines herniate out of small abdominal cavity into umbilical cord

• CRL 16–18 mm

8th week

• Digits of hand separated (but still webbed)

• Notches visible between digital rays of feet

• Stubby tail disappears

• Purposeful limb movements occur

• Ossification begins in lower limbs

• Head still disproportionately large (about half of total embryo length)

• Eye lids closing

• Ears are characteristic shape but still low-set

• External genitalia evident (but not distinct enough for sexual identification)

• CRL 27–31 mm

Ninth week to birth: fetal period

During this period, the body grows rapidly and the tissues and organs differentiate and mature (see below). The head growth rate becomes relatively slower so, by birth, the length of the head is about a quarter of the total length. Growth rate can be used to determine embryonic or fetal age (Box 9.5) and ultrasound examination can be used to examine developmental details (Box 9.6). With expert care, a fetus can be viable and may survive from 22 weeks.

Box 9.5

Estimation of embryonic/fetal age

• Greatest length (GL) is used to measure embryos of about 3 weeks, which are straight

• CRL is sitting height, used to measure older, curved embryos

• Carnegie embryonic staging system uses external characteristics to estimate developmental stage

• Number of somites

• Fetal head measurements, such as biparietal diameter and head circumference

• Abdominal circumference

• Femur length and foot length

Box 9.6

Ultrasound examination

• Estimation of size and age of embryo

• Detection of congenital abnormality

• Evaluation of growth rate

• Investigation of uterine abnormality or ectopic pregnancy

• Guidance for CVS

Box 9.7 is a summary of the changes during the fetal period.

Box 9.7

Summary of changes in the fetal period

9–12 weeks

• Growth in body length and limbs accelerates

• Ears are low-set, eyes are fused

• Primary ossification centres develop in skeleton, notably skull and long bones

• Intestines return to abdominal cavity and body wall fuses

• Erythropoiesis (formation of red blood cells) decreases in liver and begins in spleen

• Urine formation begins

• Fetal swallowing of amniotic fluid

13–16 weeks

• Rapid growth

• Coordinated limb movements (not felt by mother)

• Active ossification of skeleton

• Slow eye movements

• Ovaries differentiated and contain primordial follicles

• External genitalia recognizable

• Eyes and ears closer to normal positions

17–20 weeks

• Growth slows down

• Limbs reach mature proportions

• Fetal movements felt by mother (‘quickening’)

• Skin covered with protective layer of vernix caseosa, held in position by lanugo (downy hair)

• Brown fat deposited

21–25 weeks

• Fetus gains weight

• Skin wrinkled and translucent, appears red–pink

• Rapid eye movements begin

• Blink-startle responses to noise

• Surfactant secretion begins but respiratory system immature

• Fingernails are present

• May be viable if born prematurely

26–29 weeks

• Lungs capable of breathing air

• Central nervous system can control breathing

• Eyes open

• Toenails visible

• Fat (3.5% body weight) deposited under skin so wrinkles smooth out

• Erythropoiesis moves from spleen to bone marrow

30–34 weeks

• Pupillary light reflex

• Skin pink and smooth, limbs chubby

• White fat is 8% of body weight

• From 32 weeks, survival is usual

35–38 weeks

• Firm grasp

• Orientates towards light

• Circumference of head and abdomen are approximately equal

• White fat is about 16% of body weight, 14 g fat gained per day

• Skin appears bluish-pink

• Term fetus is about 3400 g, CRL is about 360 mm

Development of organ systems

The central nervous system

Neurulation is the formation of the neural plate and neural folds and the closure of these folds to form the neural tube, which is the precursor of the brain and spinal cord. The neural tube is completed by the end of the fourth week. The developing notochord induces the overlying ectoderm to thicken forming the neural plate, a raised slipper-like plate of neuroepithelial cells. This will give rise to the central nervous system (brain and spinal cord) and other structures such as the retina. In the middle of the third week, the neural groove appears in the centre of the neural plate (Fig. 9.10). To each side of the groove are neural folds, which enlarge at the cranial end as the start of the developing brain. Marked development of the brain is a characteristic of embryonic development in primates; human brain growth exceeds that of other species, continuing into adulthood. At the end of the third week, the neural folds start to fuse forming the neural tube, which separates from the surface ectoderm. The neural crest cells, which detach from the lateral edges of the neural folds, give rise to the spinal ganglia and ganglia of the autonomic system as well as a number of other cell types (Box 9.8). The paraxial mesoderm, closest to the notochord and developing neural tube, differentiates to form prominent paired blocks of tissue, or somites. The first somites appear from day 20. There are about 30 pairs of somites by day 30 increasing to a total of 44 pairs, but the cranial ones begin differentiation as new somites are added at the caudal end. The somites differentiate into sclerotomes, myotomes and dermatomes, which give rise to the axial skeletal bones, skeletal muscles and the dermis of the skin, respectively. The number of somites indicates the age of the embryo. The limbs carry with them the nerves from the somites from which they developed. The somatic pattern of development is important in understanding referred pain (see Chapter 13).

Fig. 9.10 The neural groove and neural tube fusion: (A) 21 days; (B) and (C) 23 days.

Box 9.8

Tissues arising from cells of the neural crest

• Spinal ganglia

• Ganglia of the autonomic system

• Adrenal medulla

• Thyroid gland

• Glial cells

• Schwann cells

• Melanocytes (pigmented)

• Pharyngeal arch cartilage

• Odontoblasts (of teeth)

• Pupillary and ciliary muscles of eye

• Dermis and hypodermis of neck and face

• Meninges

Neural tube defects (NTD) are one of the most common congenital abnormalities (see Chapter 12), resulting from the failure of the neural tube to close during embryogenesis. Neurulation begins in the middle of the neural tube and proceeds cranially (towards the head) and caudally (towards the tail). Anencephaly (absent brain) results if the neural tube fails to close in the cranial region so the brain and spinal cord are fully exposed to the exterior; this condition is lethal and most cases are diagnosed by antenatal ultrasound scans and the pregnancies terminated. Failure of neurulation at the caudal end results in spina bifida. The extent to which spina bifida results in loss of neurological function depends on the severity of the lesion and its level in the spinal cord. The most common site for spina bifida is the lumbosacral region which suggests that close of the neural tube in this region is more susceptible to environmental and genetic influences. Neurulation is very sensitive to disturbances such as teratogenic drugs or lack of folate, which is required for DNA synthesis of the rapidly dividing cells (see Chapter 7).

Most neurons are formed between 10 and 18 weeks; this is therefore the critical window for brain development. Undernutrition or other insults in the first trimester often result in microcephaly (small cranial vault) (James and Stephenson, 1998). Congenital microcephaly is also associated with environmental factors such as cytomegalovirus, rubella (German measles), or varicella (chicken pox) virus infections and toxoplasmosis as well as genetic disorders such as trisomy 21. There is also evidence that microcephaly can be a genetic abnormality. Mutations of the gene for the abnormal spindle-like microcephaly-associated protein (ASPM) can cause microencephaly. It is thought that this gene is responsible for the evolution of an increase in the size of the human cerebral cortex (Ali and Meier, 2008); a novel allele of the gene arose about the same time as humans developed agriculture and cities and began to use written language. Although mental retardation can result from genetic abnormalities and exposure to teratogens such as viruses, the leading cause of mental retardation is maternal alcohol abuse.

In later gestation, undernutrition may result in blood flow being redistributed to the brain at the expense of other tissues. As brain size in humans is proportionately larger, the effects of protecting the brain from undernutrition may be exaggerated compared with other species (Barker, 1998). Glial cells begin to develop at about 15 weeks. In the second half of pregnancy, the glial cells hypertrophy and the axons and dendrites undergo marked growth. This rapid nervous system growth spurt continues until the second post-natal year and at a slower rate for at least the first decade, and is unique to humans (Johnson, 2001). The fetal response to impaired nutrition tends to ‘spare’ the brain at the cost of somatic growth, hence the asymmetric (disproportionate) growth patterns of babies born after intrauterine growth retardation (IUGR) where their heads seem disproportionately bigger. However, neither brain function nor neurology is perfectly protected; neuronal number tends to be reduced and myelination disturbed (Gluckman and Pinal, 2003). The IUGR brain also seems more vulnerable to asphyxia.

Human brain size is enormous compared to other primates. Bite muscles of the jaw are very strong and enclose the skull in non-human primates. It is suggested that early humans developed a single gene mutation that resulted in weaker jaw muscles (Stedman et al., 2004). The slacker jaw muscles relaxed their grip on the skull, allowing the human brain to grow and expand. Alternative explanations suggest that environmental changes forced humans to invent tools and develop manual dexterity, that natural selection favoured bigger brains because they permitted more complex and supportive societies or that evolution in an environmental niche where marine food provided a good source of long chain fatty acids allowed marked brain development (Park et al., 2007). Impaired nutrition, particularly early in development, permanently affects brain size and cell number (assessed by head circumference); neurodevelopmental abnormalities occur at increased frequency in IUGR children.

Fetal sensory organs develop around the middle of gestation. At 24 weeks, the fetus responds to noise. As gestation progresses, the fetus exhibits increased sensitivity and responds to an increased range of sound frequencies. Babies are thought to enjoy being carried and cuddled because they can hear sounds of their mother's heart and digestive system, which they became accustomed to in utero.

Gastrointestinal system

The gut begins as a single tube running from mouth to anus. The mouth and anus are fused areas of endoderm and ectoderm (see above). The tube is therefore fixed at both ends so that when it grows it convolutes and loops (Fig. 9.11). Some parts of the tube dilate, such as the stomach and colon, and the gut rotates around other structures such as the developing liver. Between the sixth and eighth week of development, the proliferation of the epithelial cells lining the gut obliterates the lumen, which is then gradually recanalized. Early growth of the gut is extremely rapid so it extrudes into the amniotic cavity. If it is not withdrawn at about 10 weeks, the abdominal wall fails to close and the baby is born with exomphalos or gastroschisis. The incidence of gastroschisis is increasing; it is more common in infants born to young underweight mothers. Gastroschisis can be detected by ultrasound and by raised AFP levels in amniotic fluid and maternal serum. Normal fusion of the lateral body folds occurs at the linea nigra, the abdominal line that becomes pigmented in pregnant women (see Chapter 11). Normal growth of the gut depends on fetal swallowing. A fetus swallows about a third of the total volume of amniotic fluid per hour by the 16th week of development. Not only does amniotic fluid provide about 10% of the fetal protein requirements but it also seems to be associated with effective development of the gastrointestinal mucosa, liver and pancreas and promotion of growth.

Fig. 9.11 Convolution of the primitive gut. (Reproduced with permission from James and Stephenson 1998.)

The digestive enzymes are present from about 24–28 weeks, with the exception of lactase (see Chapter 16). Peristaltic coordination of the fetal gut is evident from the 14th week of development. By 34 weeks there is coordination of sucking, swallowing and peristalsis. As the gut matures, it produces mucus, which will eventually be required to lubricate the passage of food and faeces during transit. The mucus accumulates in the fetal gut as meconium. Adrenaline, produced in response to fetal distress, stimulates contractions of the gut and can lead to meconium-stained amniotic fluid. The liver reaches metabolic maturity relatively late in gestation, storing glycogen in the last 9 weeks. Inadequate placental transfer of amino acids will affect tissues with high protein turnover, such as the liver (James and Stephenson, 1998). The placenta is extremely metabolically active and extracts 40–60% of glucose and oxygen from the maternal circulation (Gluckman and Pinal, 2003), some of which transfers from maternal to fetal circulation and then is re-extracted from the fetal circulation. In IUGR, the rate of placental extraction from the fetal circulation increases and can lead to loss of lean body mass from the fetus and wasting. There is a hierarchy: the fetus may become catabolic to nourish the placenta and both fetus and placenta may be compromised in attempting to sustain maternal requirements. As hepatic stores of glycogen and fat are mobilized in IUGR, the liver is the first organ affected so the head:abdomen ratio is an important indicator of IUGR.

Case study 9.1 is an example of developmental abnormality of the gut.

Case study 9.1

At 11 weeks of gestation, Julie has an ultrasound scan. She is asked to return for a further scan in 2 weeks as her unborn baby appears to have some gut tissue herniating into the umbilical cord. Julie seeks advice from her midwife.

• Is this normal?

• How might you reassure Julie that the ultrasonographer was just being cautious?

• If there were a pathological condition present, what two conditions are most likely?

• How would they be further investigated before Julie is advised upon the prognosis?

The face and neck

The face is formed between weeks 5 and 12 from the brachial (pharyngeal) arches which are pouches and clefts of tissue. The endoderm of the pouches forms the parathyroid glands, thymus, tonsils and middle ear. The thyroid gland begins as epithelial cell proliferation of the tongue which descends towards the trachea. The nose grows downwards as a pillar of tissue (Fig. 9.12). The eyes, which are formed from a combination of nervous tissue and specialized ectoderm, are initially in a lateral position but move medially. The ears are initially low-set. Below the nose, maxillary and mandibular processes extend to form the floor of the nose and the roof of the mouth. The upper lip is formed from processes that extend to meet centrally. Inadequate fusion of the maxillary processes causes congenital malformations of the mouth, such as cleft lip or palate. Palatal fusion is complete by the 11th week.

Fig. 9.12 Growth of the palate and nose between the sixth and ninth week. (Reproduced with permission from James and Stephenson, 1998.)

The skeleton and skull

The skeleton develops the mesoderm layer and the neural crest. Most bones are formed initially from condensed mesenchyme tissue as cartilage which then undergoes ossification. The ribs and vertebral column develop from the sclerotome components of the somites. The skull develops from mesenchymal tissue around the brain. It is formed from the neurocranium, which protects the brain, and the viscerocranium, which forms the skeleton of the face. Each of these elements of the skull has membranous and cartilaginous components. Ossification is of the membrane rather than of cartilage and begins from the base of the skull.

The bones of the calvaria (cranial vault) have not completed development at birth. In the fetus, the flat bones of the calvaria are held together by soft fibrous sutures made of dense connective tissue, which allows some flexibility. The fetal head can mould to the shape of the maternal pelvis and distort as it passes through the birth canal. During delivery, the frontal bone becomes flat, the occipital bone is drawn out and the parietal bones overlap (this is described as ‘moulding’). The head usually returns to a normal shape a few days after delivery. Six large membranous fontanelles are formed where the sutures meet (see Chapter 13). The posterior fontanelles close at about 3 months after birth and the anterior ones close when the infant is about 18 months old. Raised intracranial pressure can be detected by palpating these fontanelles; a depression indicates dehydration.

The fetal skull is relatively large compared with the skeleton. The newborn skull has relatively thin bones compared with those in later life. The face is relatively small and has a characteristic neonatal roundish shape because the jaws are small. The paranasal sinuses (which give the individual shape of the face and resonance of the voice) are virtually absent and the facial bones are underdeveloped. After birth, brain growth is rapid so the calvaria increase markedly during the first 2 years. The calvaria continue to grow until the child is about 16 years old; the skull bones then thicken.

The muscles and limbs

The first skeletal muscles to develop are the back muscles from the paired somites. Bone formation is closely associated with muscle growth and the nervous connections from the spinal cord. The limbs become evident as buds or bulges associated with particular somites in the fourth week of development. The limb buds are formed from migration of muscle cells from the myotomes. The cells form pairs of muscle masses. Adhesive cells form a compacted region between the two muscle masses, which differentiates into cartilage. Cartilage is stiff but flexible, whereas bone is stronger but more brittle and able to fracture. Ossification, the conversion to bone structure, begins from about 8 weeks but is still not complete by birth. This preponderance of cartilage in the skeleton aids flexibility at delivery. The arms are slightly ahead of the legs in development because the fetal circulatory system gives an advantage to the upper body (see Chapter 15). Bones and muscles closest to the body develop first so the humerus and femur develop before the distal regions of the limbs. The differential timing of development means that drugs such as thalidomide affect different limbs and parts of the limbs depending on time of exposure of the fetus to the teratogens (Box 9.9). By 41 days, the fingers and toes develop from paddle-like plates. The sculpting of the digits is due to apoptosis of the tissue between the digital rays. A common minor congenital defect observed at birth is a failure of separation of the digits. By 9 weeks, the body skeleton is almost complete, although the skull bones are still forming. Development of the limbs and digits can be impaired by amniotic bands (derived from tears in the amnion possibly as a result of infection or toxic insult) which encircle and constrict parts of the fetus.

Box 9.9

Thalidomide

Thalidomide was marketed in 1957 as ‘Contergan’, a sedative and antiemetic drug suitable for treating nausea and vomiting of pregnancy (morning sickness). Congenital malformations of the limbs and ears rose in parallel with sales of the drug with a lag of 7–8 months (Lenz, 1962; McBride, 1961). The drug was withdrawn in November 1961; 5850 infants were affected, of whom 40% died, leaving 3900 survivors. Thalidomide disturbs cartilage formation and the establishment of the nerve connections to muscles. It is teratogenic 20–36 days after fertilization. Limb development begins at 24 days. Early exposure to thalidomide caused unique birth defects including absence of arms; later exposure successively affected development of ears, legs and thumbs. Absence of limbs is called amelia and absence of long bones, so a hand or foot comes directly from the torso, is called phocomelia. Different species metabolize the drug differently; fetal development in the rodent test species (rats and mice) was not affected. Thalidomide and its analogues have effects on inflammation and blood vessel growth; it is currently used for treatment of complications of leprosy and has been used in clinical trials for a range of disorders including cancers and AIDS (Melchert and List, 2007).

The cardiovascular system

This is one of the first systems to develop; its function is important extremely early in development, unlike some of the other systems that do not have to achieve full function until after birth. This is because, as the embryo becomes larger, diffusion of oxygen and nutrients is no longer adequate.

The primordial vascular system develops by vasculogenesis (new vessel formation) and later development is by angiogenesis (new vessels branching off the existing vessels and remodelling). A few cells in the mesoderm of the yolk sac lose adherence and start to move, forming clusters called blood islands (Fig. 9.13). The haemocytoblasts (or haemangioblasts), the precursors of both blood cells and blood vessel endothelial cells, are nucleated and start to synthesize primitive forms of haemoglobin. The outer cells of the blood islands, angioblasts, develop characteristics of endothelial cells, the cells that line blood vessels. The blood islands fuse, forming vascular channels that eventually amalgamate to form a primitive vascular network with identifiable routes. Blood vessels form by vasculogenesis, where the vessels develop from blood islands, and by angiogenesis, where new vessels branch off existing vessels. The endothelial cells interact with pericytes, vascular smooth muscle cells, to form the vessel walls (Bergers and Song, 2005). The organization of the routes across the yolk sac is similar to the geographical organization of river deltas where little streams meander and combine, taking the route of least resistance.

Fig. 9.13 Formation of the first blood vessels: (A) appearance of blood islands; (B) vessels at 24 days ((A) Reproduced with permission from Fitzgerald and Fitzgerald, 1994; (B) reproduced with permission from Goodwin, 1997.)

Expansion and elastic resistance of the vessel walls, which become rhythmic generating a peristaltic pattern, propel the blood cells. Blood vessel differentiation and growth is orchestrated by a range of signals including cell adhesion molecules, transcription factors and angiogenic growth factors and their receptors (Breier, 2000). It is thought that abnormal molecular regulation of blood vessel formation is the cause of capillary haemangiomas, the most common tumours of the infant. Capillary haemangiomas are dense masses of capillary endothelial cells which are usually associated with craniofacial structures. They are usually benign and self-limiting but occasionally they may proliferate or ulcerate and bleed or may persist as ‘port-wine stains’.

The primitive heart develops from a horseshoe area of embryonic mesoderm, anterior to the prochordal plate. It forms two tubes, one on each side of the foregut, which fuse to form a single heart tube. The primitive atrium forms where the flow from the umbilical veins from the placenta joins with the blood vessels from the head, generating the greatest volume of blood. The swirling vortex of blood leaving the primitive atrium induces the development of the primitive ventricle, which becomes the main source of pumping activity. The characteristic shape of the heart is generated by the flow of blood cells within the vascular channels; this causes the heart tube to form an S-shaped loop that will eventually take on the configuration of the heart (Fig. 9.14). By 21 days after fertilization, the cells surrounding the heart have become differentiated as myocardial cells capable of eliciting an organized response, so the heart, which consists of four chambers in series, begins beating.

Fig. 9.14 Formation of the heart from bending of the cardiac tube (21–35 days) and formation of the heart chambers.

The development of the outer layers of the vessel walls is stimulated by stress (Martyn and Greenwald, 1997). In areas where there is more turbulence, the vessel wall responds by developing more elasticity. Therefore, the heart and arterial structures develop thicker and more elastic walls. The mature organization of the chambers of the heart is achieved by the ingrowth of the septa towards the central atrioventricular endocardial cushion in the centre and apoptosis of excess tissue (Fig. 9.14). Abnormalities in the endocardial cushion development or excess apoptosis result in cardiac malformations including atrial and ventricular septal defects (known as a ‘hole in the heart’) and defects or transpositions of the great vessels. Abnormalities of the cardiovascular system are the more common human birth defect partly because they often allow normal fetal development and only become significant from birth. Most cardiovascular abnormalities have multifactorial causes due to the interaction of environmental and genetic factors. Cardiovascular teratogens include vitamin A, alcohol and some viruses and maternal gestational disorders such as hypertension and diabetes are associated with an increased risk. Many chromosomal abnormalities and genetic syndromes cause heart malformations probably because so many different genes are involved in the complex embryonic development of the cardiovascular system.

The growth of the fetal heart partially depends on afterload. If afterload is increased by factors leading to peripheral vasoconstriction or high placental impedance, the likely outcome is a growth-restricted baby with an enlarged heart (Veille et al., 1993). If the fetus receives less than adequate nutrition or oxygenation during its development then blood flow is diverted to the brain and heart. The decreased flow to the peripheral vessels results in the development of less elastic tissue, which is the hypothesis underlying the association of poor maternal nutrition with an increased risk of cardiovascular disease in adult life (Barker et al., 1993). The initial response to impaired nutrition is to increase placental growth; if this is not adequate, blood flow is diverted to the brain (and other essential organs such heart, adrenal glands and placenta). Therefore, adults, who were small at birth, but with relatively large placentas, have an increased risk of developing hypertension because less elastic tissue was established in their blood vessels during fetal development (Fig. 9.15). It is argued that impaired fetal nutrition also leads to fewer tissue stem cells being formed which leads eventually to early exhaustion of organ function and the development of chronic adult disease (Cianfarani, 2003).

Fig. 9.15 The fetus adapts to suboptimal nutrition with strategies for survival. The slower growth rate reduces use of nutrients but affects final organ size and function. The redistribution of blood to the brain affects development of the blood vessels and predisposes to later hypertension. Altered metabolism and peripheral insulin resistance favours glucose availability for the developing brain but the ‘thriftiness’ predisposes to insulin resistance, obesity and type II diabetes if the individual experiences good or over-nutrition in later life.

The respiratory system

The trachea and major bronchi develop as outpouches of the primitive alimentary tract. The development depends on interaction between the endodermal bud from the developing foregut and the splanchnic mesoderm it invades at about day 22. The bud bifurcates between day 26 and day 28. In the fifth week of development, three secondary buds develop on the right branch and two on the left; these are the main bronchi and primitive lobes of the lung. There are four stages in the development of the respiratory system: the embryonic phase from weeks 3 to 5, the pseudocanalicular phase from 5 to 16 weeks, the canalicular phase from 16 to 26 weeks and the terminal sac phase from 24 weeks until birth (Fig. 9.16). One of the most critical stages in development is the production of phospholipid-rich surfactant from the type II pneumocytes, allowing efficient inflation and gas exchange following birth and therefore post-natal survival. Although the cells can be identified at about week 22, production of surfactant increases significantly after 30 weeks. The diaphragm starts to develop as the peritoneal membrane high in the neck and descends as the lungs and heart develop. This is the reason why diaphragmatic pain is often felt as referred pain in the shoulder. If the development of the diaphragm is incomplete, it results in a diaphragmatic hernia which allows the abdominal contents can protrude into the chest cavity. In severe cases, the abdominal contents can restrict growth and development of the lungs (causing pulmonary hypoplasia).

Fig. 9.16 Respiratory system development: (A) pharyngeal pouches (4 weeks); (B) 32 days; (C) 35 days; (D) pseudocanalicular phase (17 weeks); (E) canalicular phase (17–26 weeks); (F) terminal sac phase (26 weeks).

Fetal breathing movements (FBM) are a feature of normal fetal life and are used to assess fetal well-being. FBM oppose lung recoil and maintain the level of lung expansion that is essential for normal growth and structural maturation of the fetal lungs (Harding and Hooper, 1996). Normal development of the fetal lungs is very important as there is limited capacity for later recovery. FBM are inhibited by fetal hypoxaemia, hypoglycaemia, maternal alcohol consumption, maternal smoking, intra-amniotic infection and maternal consumption of sedatives or narcotic drugs. The absence of FBM is associated with lung hypoplasia, premature rupture of fetal membranes and oligohydramnios. Decreased amniotic fluid volume causes decreased lung growth and expansion, whereas tracheal occlusion, which prevents expulsion of lung fluid, can cause overgrowth of lung tissue (Nardo et al., 1998). At term, the infant has about 50 million alveoli, half the adult number; these continue to increase in the first 8–10 years of life.

The urinary system

The urinary and genital systems both develop from the intermediate mesoderm and are closely associated (see Chapter 5 for a description of genital development). During embryonic folding, urogenital ridges appear each side of the primitive aorta (Fig. 9.17). The nephrogenic ridge develops into the renal system of kidneys, ureters, bladder and urethra. Abnormalities of the kidneys and ureters affect 3–4% of newborn infants. Most of the abnormalities are harmless, such as variation in blood supply, abnormal position or shapes, and urinary tract duplications such as supernumerary kidneys. However, unilateral renal agenesis (one kidney failing to develop) affects 1 in 1000 liveborn babies. Bilateral renal agenesis or Potter's syndrome (inadequate development of both kidneys) affects 1 in 3000 fetuses and is incompatible with life. It is usually associated with oligohydramnios. Three pairs of kidneys develop during fetal development: the pronephroi, the mesonephroi and the metanephroi (singular: pronephros, mesonephros and metanephros). The pronephroi are transient non-functional structures that exist for only a few weeks. When they degenerate, their ducts are utilized in the next stage. The mesonephroi appear in the fourth week and function as intermediate kidneys until the end of the embryonic period, disgorging waste products into the remnants of the yolk sac. They degenerate and disappear in the eighth week, although parts of their structure persist as mesonephric or Wolffian ducts in males (see Chapter 5).

Fig. 9.17 The development of the renal system from the urogenital ridges.

The permanent kidneys, or metanephroi, develop from the fifth week and begin to function about 4 weeks later. The kidneys start development in the pelvis and appear to migrate upwards. In fact, this observation is due to continued downward growth of the embryo. As the kidneys ‘ascend’ out of the pelvic area, new arteries at successively higher levels supply them. During fetal life, the kidney is subdivided into lobes, which disappear in infancy as the nephrons grow. The main increase in size is due to elongation of the proximal convoluted tubules and loops of Henlé. Disruption of renal branching during development leads to renal dysplasia, the major cause of renal failure in children (Piscione and Rosenblum, 2002). Functional maturation of the kidneys occurs after birth.

Until 20 weeks' gestation, the skin is not keratinized so fluid can move through this semipermeable membrane. Essentially the outer barrier is the amnion. As the skin matures and lays down keratin, the rate of transudation decreases and the outer barrier of the fetus becomes the skin. The urine then becomes an important source of amniotic fluid. The fetus produces up to 600 mL of urine per day. Amniotic fluid is also produced by the amniotic membrane and the fetal lungs. The fetus swallows most of the amniotic fluid; the rest diffuses through the amniotic membranes to the maternal circulation.

The epidermis of the skin develops from the ectoderm, which is colonized from melanocytes from the neural crest and Langerhans cells from the bone marrow. The dermis is derived from the embryonic mesoderm.

Fetal growth

At 4 weeks, the crown-rump length (CRL) is about 4 mm and increases by 1 mm per day up to 30 mm (Beck, 1996). Thereafter, between weeks 8 and 28, the growth increases markedly, to about 1.5 mm per day, so this period is recognized as the fetal growth period. The organs and tissues continue to grow and mature. Although growth is most rapid during this period (compared with any other time in life), factors affecting growth may have their origins earlier. In fact, environmental insults regulating growth appear to last for several generations. Fetal growth is due to interaction between the genetic drive for growth and the nutritional supplies in pregnancy to support it, which involves a dynamic interaction between fetus, placenta and mother.

There are characteristic differences between the different phases of growth during development (James and Stephenson, 1998). Growth in the first trimester is principally through increased cell number. In the second trimester, cell division continues albeit at a slower rate and the cells increase in size. In the third trimester, cell division slows further and the increase in cell size continues. There is little variation in fetal growth up to about 16 weeks of gestation (Gluckman and Pinal, 2003), after which genetic and environmental influences have a more marked effect on outcome. Fat deposition, determined by nutrient availability and insulin levels, plays an important contribution in the final weight. There are 42 successive cell divisions between fertilization and birth, but only five more from birth to adult size (excluding mitotic cell division to replace dead cells).

All low-birthweight infants are potentially at some health risk. Low birthweight is traditionally defined as being less than 2500 g at birth. Low birthweight remains one of the great challenges to modern health care services; small size at birth can affect susceptibility to infection, rate of post-natal growth and neurocognitive development. Low birthweight is associated with increased fetal mortality as well as higher neonatal and infant morbidity and mortality with the most adverse outcomes arising in the most immature infants. A number of chronic adult diseases originate in utero as a result of fetal adaptation to suboptimal quality or quantity of nutrients in order to optimize survival (Langley-Evans and McMullen, 2010), including glucose intolerance, insulin resistance and type II diabetes mellitus, heart disease, hypertension, stroke, obstructive lung disease, hyperlipidaemia, hypercholesterolaemia, hypercortisolaemia, renal disease, osteoporosis, schizophrenia, obesity and reproductive disorders.

The use of low birthweight as the outcome measure of the success of a pregnancy is very widespread and it can be measured with precision and validity (Kramer, 1998). Infant mortality increases exponentially with lower birthweight. However, birthweight is a function of two factors (gestational length and rate of fetal growth) which have different aetiologies and different prognoses. The simple definition of a low-birthweight infant as one who weighs less than 2500 g at birth does not differentiate between infants who are growth-restricted (small at term) and infants who are born prematurely. Prematurity is often complicated by IUGR. Some small-for-gestational age (SGA) infants are constitutionally small rather than growth-restricted or growth-retarded. Growth in the first trimester of pregnancy affects birthweight; early growth restriction is associated with complications and increased risk of adverse outcomes (Bukowski et al., 2009).

Birthweight is a crude outcome measure of optimal intrauterine growth and development; suboptimal maternal body composition and nutrient intake can have a long-term effect on the offspring without necessarily affecting size at birth (Godfrey, 2001). Birthweight does not identify effects of nutrition on body composition and development of specific tissues and organs; a similar birthweight can be attained with different growth trajectories. Birthweight may not identify growth restriction. For instance, if an infant does not reach its potential birthweight but is born above 2500 g, it will not be classified as growth-restricted. Conversely, an infant with ‘normal’ birthweight, such as 3.4 kg, may be growth-retarded and have long-term health risks if it was destined to be bigger under optimal intrauterine conditions. Most infants born of low birthweight in developed countries are born prematurely rather than growth-restricted; in most cases, the cause(s) of preterm delivery is not known. Rates of low birthweight babies are higher in areas with a higher level of socio-economic deprivation; birthweight is also related to income level and accessibility to healthcare, education and housing.

A variety of factors affect fetal growth (Box 9.10). Chromosome and genetic disorders often cause fetal growth retardation; excluding these, the dominant cause of growth retardation is due to an inadequate supply of nutrients and oxygen (Gluckman and Pinal, 2003) related either to maternal supply or to placental transfer capacity.

Box 9.10

Factors affecting fetal size

• Fetal factors

• Maternal size (lean body mass): maternal genetic effects

• Maternal weight gain and nutrition in pregnancy

• Maternal age extremes

• Maternal behavioural factors such as smoking, recreational drug use

• Multiple pregnancy

• Fetal oxygenation: affected by maternal anaemia and so on

• Maternal medical conditions such as hypertension, heart disease, infections and diabetes

• Placental sufficiency affected by pre-eclampsia, uterine blood flow and so on

• GHs such as insulin and IGFs

Fetal factors

Male offspring are on average heavier than female offspring. The ovaries have limited capability to synthesize steroid hormones, whereas the testes produce testosterone, which has anabolic effects. Studies suggest that the Y chromosome and higher testosterone level positively promotes growth (James and Stephenson, 1998). There may also be sex-specific adaptation of the placenta which affects fetal growth (Clifton, 2010). Multiple pregnancies tend to result in smaller babies, probably because of the limited haemodynamic support in late gestation, although overcrowding is also implicated. Aneuploidy is also a risk factor for IUGR (see Chapter 7). Parity affects birthweight; first-born infants tend to be slightly lighter than second and subsequent siblings (Shah, 2010a).

Maternal size

The classic experiments on horses showing that the size of offspring of hybrid crosses between small Shetland ponies and large carthorses was most closely related to maternal size demonstrated that maternal size is a critical determinant of birthweight (Walton and Hammond, 1938). However, final adult size is also affected by paternal genetics to different degrees in different species; in humans about 5% of the variability in size is attributed to paternal influences (Robinson and Owens, 1996). In the past, pregnant women were asked for their shoe size, which correlates with their pelvic size, and their husband's hat size, which was found to be related to the size of the baby; the relationship of the two sizes was used to predict the likelihood of difficulties in delivery. A small maternal size appears to impose a constraint on fetal growth, although factors such as immaturity, social circumstances, maternal behaviour (such as smoking and alcohol consumption), diseases and psychological stress all affect the outcome of the pregnancy. Shorter maternal stature is positively correlated with lower socio-economic status, malnutrition, chronic disease, increased levels of stress and large family size. The cycle of deprivation tends to repeat, as there is a correlation between maternal height and birthweight and between birthweight and adult size, which is not solely due to genetic influences.

Half-siblings who share the same mother have similar birthweights (Gluckman and Harding, 1994). The birthweights of babies born after ovum donation are more strongly related to the weight of the recipient mother than to that of the donor woman (Brooks et al., 1995). These findings suggest that birth size is more strongly influenced by uterine environment than by genetics.

It is suggested that there is a conflict between the maternal and paternal genes governing fetal size (Moore and Haig, 1991). Paternal genes favour fetal growth and the transfer of nutrients to the parasitic fetus; if this happens at the expense of maternal health or life, the male can choose a different mate. Maternal genes limit transfer to the fetus to optimize survival of the mother and her children. The father's birthweight influences placental size. Birthweights of mothers correlate with their children's birthweights and even with their grandchildren's birthweights, suggesting that the maternal constraint on fetal growth is set very early (Barker, 1998). The paternal effect on the fetal growth trajectory is permitted by the lifting of the maternal constraint on growth. Therefore, fetal growth rate responds to, and is appropriate for, the prevailing nutrient availability. Maternal constraint is also a mechanism for limiting fetal growth to maternal pelvis size. Constraint of growth by maternal factors is important in preventing fetal overgrowth and dystocia which is risky for mother and infant (and survival of the species). There are paternal factors that influence birthweight; infants whose fathers are older or who were born of low birthweight themselves have an increased chance of having lower birthweight (Shah, 2010b).

Growth hormones

It has been suggested that growth in children from fetus through infancy and childhood to puberty follows a mathematical model on which three growth curves are imposed, forming a sigmoidal curve (Fig. 9.18; Karlberg et al., 1994). Phase 1, the infancy growth rate, begins in fetal life with a rapid deceleration until about 3 years of age. This is the phase of growth that seems to be regulated by insulin-like growth factors (IGFs; see Chapter 4); the effect of poor nutrition on growth may be mediated by IGFs. The childhood phase begins in the first year of life and is due to the effect of growth hormone (GH; see Chapter 3), provided thyroid hormone secretion is normal. During this period most of the growth is localized in the lower body (particularly leg length), as the long bones are very sensitive to GH. Children who have deficiency of GH or who are encephalic have normal birthweight and early infant growth; the deficiency usually becomes apparent only after 6 months of age. The final component of growth is the pubertal growth spurt, which is stimulated by the interaction of sex hormones with GH. Although levels of GH in the fetus are high, GH receptors are expressed at low levels in fetal tissues so GH has little effect. This is supported by observations that GH-deficient fetuses or young infants have almost normal linear growth. Fetal growth seems to be controlled rather by IGFs and their receptors (Robinson and Owens, 1996).

Fig. 9.18 Karlberg's model of growth: (A) sigmoidal curve from a combination of three growth curves; (B) the curve of weight increase; (C) the curve of increase in length. (Reproduced with permission from Karlberg et al., 1994.)

IGFs are mitogens (i.e. they stimulate cell division and differentiation) and are modulated by binding proteins, which control growth before and after birth (see Chapter 15). This mechanism can explain both the interaction of genetic drive and nutrient supply and the effects of maternal and paternal size. IGF-II is the primary growth factor influencing embryonic growth, whereas IGF-I, produced by the fetal liver and amniotic membranes, regulates growth in late gestation. IGF-I increases the efficiency of the placenta so fetal weight increases without a corresponding increase in placental weight. Fetal insulin, driven by fetal glucose availability, is the main regulator of IGF-I; IGF-I is very sensitive to maternal nutrition. IGF-I is also involved in fetal gut maturation. Both IGF-I and IGF-II have anabolic effects via the type I receptor. IGF-II also binds to the type II receptor, which effectively competes with the type I receptor for available IGF-II. IGF-II is paternally imprinted and expressed and promotes growth via the IGF-I receptor, whereas the type II receptor, which is hypothesized to be maternally imprinted, limits or controls growth by clearing or ‘mopping up’ the free IGF-II (Harding and Johnston, 1995). Overexpression of IGF-II leads to the Beckwith–Weidemann overgrowth syndrome. Fetal hyperinsulinaemia appears to inhibit the production of IGF-I-binding protein, thus lifting the restriction on IGF-I and contributing to macrosomia (Wang and Chard, 1992). Nutrient availability may promote IGF-I levels and fetal growth. Thus, a balance can be achieved between paternal genes promoting growth and maternal genes restricting and regulating growth.

Insulin itself has a growth-promoting effect in the fetus; it is a growth-promoting hormone which signals nutrient plenty. Its somatogenic (growth-promoting) actions are mediated via IGF-I but it has a direct effect on fat deposition. Maternal hyperglycaemia causes increased placental transfer of glucose to the fetus. The higher concentration of glucose stimulates the fetal pancreas to produce insulin, which facilitates cellular uptake of glucose, stimulating anabolic metabolism and fetal growth. Babies of diabetic mothers are often macrosomic owing to particularly large fat stores. The macrosomia tends to be somatic so the bodies of macrosomic infants are big so their heads can appear relatively small. Fetal insulin deficiency, which is rare, can occur with nutrient deprivation or pancreatic agenesis, and results in fetal growth retardation (symmetrical IUGR) and decreased levels of body fat and muscle development (Robinson and Owens, 1996).

Glucocorticoids affect fetal growth and maturation by altering production and secretion of hormones, regulating receptor density and altering the activity of enzymes involved in activating and deactivating hormones (see also Chapter 15). The fetus is usually protected from maternal glucocorticoids because they are inactivated by the placental enzyme 11β-hydroxysteroid dehydrogenase which effectively acts as a barrier. However, maternal undernutrition downregulates this enzyme so the fetus is exposed to increased glucocorticoids (Gluckman and Pinal, 2003), which leads to fetal growth maturation. However, the higher levels of cortisol enhance maturation of the lungs and other organs, promoting survival of the IUGR infant; premature delivery frequently accompanies IUGR. Thyroid hormones promote fetal development and signal energy availability.

Maternal nutrition

Fetal growth is related to maternal size (reflecting nutrient level during her own fetal development), maternal body composition (which indicates nutrient supply), nutrient availability in pregnancy and placental efficiency. If periconceptual maternal nutrition sets the growth trajectory early in gestation, the fetal growth rate is more likely to be accommodated by nutrient availability when its demands are high in later gestation. The fetus is able to adapt metabolically to undernutrition in pregnancy by altering its growth rate and sparing nutrients for certain tissues, like the brain. This can lead to disproportionate organ development and fetal growth patterns; fetal adaptations to undernutrition tend to be permanent (Barker, 1998).

Maternal nutrition stores correlate with birthweight. Pregnant women exposed to conditions of starvation during the famines during World War II were particularly susceptible to nutrient deficiency if they were subject to nutritional deficiency in the preconceptual period or in early pregnancy. Not only was the size of their babies significantly smaller, but the prenatal growth of their grandchildren was also affected (Lumey, 1992). Nutrient deprivation later in pregnancy affected birthweights and fat deposition, but the lengths of the neonates were not affected so much and the babies appeared to regain normal weights after birth. However, the metabolic adaptation in those fetuses exposed to nutrient deprivation late in gestation is associated with persistent insulin resistance and a marked trend to develop glucose intolerance and non-insulin-dependent diabetes mellitus (NIDDM) later in life (Phillips, 1996). Fetal nutrition is not directly related to maternal nutrition; placental dysfunction can limit transfer of nutrients from the mother to fetus (Sibley, 2009).

In humans, dietary intervention studies have had disappointing results, producing improvement in fetal growth only in severely undernourished women (see Chapter 12). Part of the problem may be the methodology and ensuring that supplements intended for pregnant women are not used as alternative sources of nutrition or to feed other members of the family. The timing of the nutritional supplements may also be important, as fetal growth trajectories may be set before the nutritional status of the mother is improved by the intervention. Women experiencing marginal diets and seasonal famine for generations appear to have evolved strategies to conserve energy by suppressing metabolic rate and acquiring little fat during the pregnancy (Durnin, 1987). The energy cost of pregnancy in affluent countries where food is plentiful may be met with little or no increase in energy intake, although economies in energy expenditure may offset the increased requirements (see Chapter 12). Prepregnant size (body fat levels) may direct the trajectory that sets fetal growth via leptin, the product of the ob gene (Rink, 1994). Appropriate conditions during pregnancy can then fulfil the requirements for this trajectory to be achieved. In experimental animals, the effect of moderate maternal malnutrition over a number of generations is decreased birthweight, which is maintained for a few further generations even when food supply is restored to a good level (Stewart et al., 1980). A plentiful food supply imposed after generations of malnutrition in these animals is associated with obstructed labour and poor fetal outcome.

Acute undernutrition in late gestation can cause premature delivery by stimulating signals which promotes cervical ripening and uterine contractility and cause early labour (see Chapter 13). Macronutrient balance at different stages of pregnancy may affect fetal growth (see Chapter 12). Micronutrient availability can affect the somatotrophic and insulin regulation of growth; zinc deficiency is associated with IUGR. Vitamin E and vitamin A affect insulin sensitivity and GH secretion, respectively. Folate status affects gene imprinting and methylation.

Maternal nutrition could also exert an effect on fetal growth even before fertilization. The nutritional support of follicular development prior to ovulation and fertilization may affect the growth trajectory of the embryo (see above). Nutrition of the embryo prior to implantation may be important, as demonstrated by in vitro fertilization (IVF). Sheep and cattle embryos from IVF that are cultured for a few days before being replaced in the uterus grow into significantly larger fetuses (James and Stephenson, 1998). It is not clear whether IVF affects human birthweight. However, some ‘unexplained infertility’ in humans appears to be related to an inadequately developed endometrial lining. Couples with a previous history of ‘unexplained infertility’ have a high rate of small-for-dates infants, which may be associated with poor conditions for implantation (Wang et al., 1994).

Maternal behaviour

Differences in birthweight across different socio-economic groups may be largely attributable to differences in cigarette smoking. The birthweight appears to fall by about 14 g multiplied by the average number of cigarettes smoked per day. Smoking is associated with a poorer diet and level of health care, although effects on oxygen transfer are probably compensated for by 2,3-bisphosphoglycerate (2,3-BPG), which improves the efficiency of oxygen–haemoglobin dissociation (see Chapter 1). Smoking causes nicotine-induced vasoconstriction of the uterine vessels, carbon monoxide inhibits oxygen diffusion and cyanide affects enzyme systems (James and Stephenson, 1998). Improving fetal oxygenation in conditions of maternal hypoxia has achieved an improved fetal outcome (Battaglia et al., 1992) but there is controversy about the effect of maternal iron deficiency on pregnancy outcome (Godfrey et al., 1991). Alcohol consumption in excess of 40 mL/day is associated with effects on growth; one of the common effects of fetal alcohol syndrome is growth retardation (Hannigan and Armant, 2000). The use of hard drugs, such as heroin and cocaine, in pregnancy is associated with low birthweight babies but again it is difficult to dissociate the use of the drugs from the other variables. Caffeine, whether from coffee or other sources such as soft drinks, has an effect on fetal growth (see Chapter 12).

Other factors affecting fetal growth

Medical complications of pregnancy or pre-existing maternal diseases can affect fetal growth. Mild maternal hypertension does not restrict growth but severe hypertension is associated with low birthweight particularly if it is complicated with renal disease. Pre-eclampsia is a major cause of low birthweight; it has been suggested that IUGR of unknown cause may be due to undiagnosed pre-eclampsia (see Chapter 8). Obese and overweight women appear to be at increased risk of delivering a preterm infant (McDonald et al., 2010a). Severe respiratory and cardiovascular problems and chronic renal disease are also associated with growth retardation. Fetal growth retardation has also been observed in women with congenital uterine abnormalities. Infants conceived by IVF are more likely to be preterm and of low birthweight even when multiple gestation is taken into account (McDonald et al., 2010b).

Fetal malformations, especially those due to chromosomal abnormality, are strongly correlated with impaired growth rates. Trisomies and Turner's syndrome have a marked effect on birthweight (James and Stephenson, 1998). Chorionic villus sampling (CVS) indicates that in 1–2% of conceptuses tested there is a degree of confined placental mosaicism (where one or more types of placental cells have nuclei with an abnormal number of chromosomes). Placental mosaicism is associated with an increased frequency of IUGR (Robinson and Owens, 1996).

Maternal and fetal infections, such as rubella and cytomegalovirus, also detrimentally affect growth. It is not clear whether HIV affects fetal growth as coexisting problems cannot be dissociated. In developing countries, malaria infection causes placental disease and affects fetal growth. The fetus is also at risk from certain types of anti-malaria drugs such as quinine taken by the mother. Placental supply of amino acids is close to the minimum required to support fetal protein synthesis. It is possible that the adverse circumstances limiting fetal growth do so by increasing levels of catabolic hormones, such as catecholamines, cortisol and β-endorphin (Robinson and Owens, 1996) or by altering the expression of the receptor for IGF-II (Haig and Graham, 1991).

Complications associated with SGA

Babies who are small for gestational age (SGA) have an increased risk of perinatal complications (Box 9.11). Although some catch-up growth may occur post-natally, some of the effects of IUGR may be irreversible (Barker, 1998).

Box 9.11

Complications associated with SGA

• Increased mortality

• Short- and long-term pulmonary morbidity

• Intrapartum hypoxia

• Hypothermia

• Hypoglycaemia

• Necrotizing enterocolitis

• Ophthalmic morbidity

• Neurological morbidity

• Delayed psychomotor development

• Polycythaemia

• Infection

• Pulmonary haemorrhage

• Sudden infant death syndrome

• Adult-onset cardiovascular and metabolic disease

Case study 9.2 details the example of a baby with low birthweight.

Key points

• During the second week of development, the inner cell mass differentiates into the bilaminar disc, consisting of two germ layers: the epiblast and the hypoblast. The definitive yolk sac is created and the amniotic and chorionic cavities are evident. The differentiated cells migrate and adhere and the genes are switched on and off.

• The embryonic period consists of cell growth (increased cell number and size), differentiation, organogenesis (organization of tissues into organs) and morphogenesis (development of shape). This is the period that is most susceptible to teratogens, which can cause major morphological abnormalities.

• Gastrulation is the major event of the third week. It begins with the appearance of the primitive streak, results in the conversion of the bilaminar disc into a trilaminar disc, consisting of ectoderm, mesoderm and endoderm and establishes the axis for further embryonic development. The neural tube, precursor of the nervous system, and the somites also appear in the third week.

• The trilaminar disc is converted into the characteristic vertebral structure by differential growth of the cell layers causing folding and fusion.

• Weeks 4–8 are the period of organogenesis, differentiation of the major organ systems.

• Fetal growth is influenced by genes and the environment, but limited by nutrient and oxygen supply. Paternal genes tend to favour fetal growth whereas maternal genes tend to constrain fetal growth to a growth trajectory that may be set by environmental influences prior to fertilization.

• The fetus can adapt to undernutrition by altering metabolism and blood flow to protect the brain, albeit at the expense of other organs.

• Stem cell collection and storage from cord blood samples obtained immediately after delivery is becoming increasingly popular. Careful consideration is required by all health professionals, if this is requested, to ensure all legal and ethical issues are considered. Currently, it is still unclear what the actual benefits of fetal stem cell storage; however, on-going research is progressing into exploring the possible advantages of this. It is possible that some genetic conditions may be treatable with donated stem cells extracted from cord blood such as metabolic, immune and haematological disorders.

Application to practice

• An understanding of fetal development is required in the explanation of congenital conditions.

• Many factors, some of which are modifiable and can be affected by advice and guidance of the midwife, for example maternal smoking, stress and nutrition, affect fetal development and growth.

• As pregnancy progresses, most women are keen to know how their baby is developing, so the midwife should be able to describe fetal development and growth in an appropriate way.

• A basic understanding of fetal development is important in recognising abnormal conditions in the physical examination of the newborn (see Chapter 15).

Case study 9.2

Razia gives birth to a healthy female infant at term. The baby appears healthy and chubby although she weighs only 2.6 kg.

• Is the midwife right to assume that Asian babies are normally smaller than Western babies?

• What reasons would you give to argue for or against this assumption?

Annotated further reading

Barker, D.J.P., Mothers, babies and health in later life. ed 2 (1998) Churchill Livingstone, New York .

Epidemiological studies link a number of adult diseases with fetal development. This book examines the evidence that fetal adaptation to undernutrition irreversibly alters anatomic, physiological and metabolic development, and links the fetal origins hypothesis to health policy.

Burton, G.J.; Barker, D.J.P.; Moffett, A., The placenta and human developmental programming. (2010) Cambridge University Press, Cambridge .

An edited textbook with contributions from scientific experts which explore the role of the placenta in developmental programming, how placental development can disrupt the placental supply of nutrients and how gene expression can be affected by environmental, immunological and vascular insults.

Carlson, B.M., Human embryology and developmental biology. ed 4 (2008) Mosby, St. Louis .

A well-illustrated textbook which covers the molecular basis of development, cellular aspects, developmental anatomy and the progression of development. Includes recent research findings, case studies, timeline information, review questions and useful end-of-chapter summaries.

Copp, A.J.; Greene, N.D., Genetics and development of neural tube defects, J Pathol 220 (2010) 217–230.

An excellent up-to-date-review covering mammalian neurulation and congenital defects of neural tube closure in depth, including the underlying developmental mechanisms, how mutant genes disrupt neurulation, gene-gene and gene-environment interactions, the mechanisms by which folic acid supplementation prevents neural tube defects and possible causes of folic acid-resistance.

Gluckman, P.D.; Hanson, M.A., The fetal matrix: evolutions, development and disease. (2004) Cambridge University Press, Cambridge .

Written by two of the experts in this field, this book covers fetal programming, evolutionary mechanisms that promoted survival of our hunter–gatherer ancestors, the implications of nutritional transition, and directions for future research and treatment.

Gluckman, P.D.; Hanson, M.A., Developmental origins of health and disease. (2006) Cambridge University Press, Cambridge .

A well-referenced edited book that describes the epidemiological studies and animal research work that led to the current understanding that subtle influences on the fetus and during early life have profound and irreversible consequences for adult health and the risk of a wide range of diseases; includes a review of the key concepts of evolutionary developmental (‘evo-devo’) biology.

Sadler, T.W., Langman's medical embryology. ed 11 (2009) Lippincott Williams Wilkins, Philadelphia .

An up-to-date text (supported by online resources), illustrated with excellent line-drawings and photographs which covers stages of human development in detail with timelines and sections on the interaction between genetics and human development; links molecular aspects including cellular signalling and experimental principles to clinical correlates.

Moore, K.L.; Persaud, T.V.N., Before we are born: essentials of embryology and birth defects. ed 7 (2007) Saunders, Philadelphia .

This book covers normal and abnormal human development week by week from fertilization through the development of the major organs and physiological systems to birth.

Moore, K.L.; Persaud, T.V.N., The developing human: clinically orientated embryology. ed 8 (2007) Saunders, Philadelphia .

This book is a more detailed description of embryological development, targeted at clinicians, which covers new research findings and their clinical applications. It includes aspects of molecular biology, effects of teratogens and detection of fetal defects.

NIH National Institutes of Health, Stem Cell Basics. (2010) U.S. Department of Health and Human Services ; http://stemcells.nih.gov/.

A useful primer about stem cells which describes the biological properties of stem cells, current research areas and the potential use of stem cells in research treating disease; includes a comprehensive glossary.

Nüsslein-Volhar, C., Coming to life: how genes drive development. (2006) Kales Press .

A unique portrayal of developmental biology, by an inspiring Nobel prize winner, which covers the historical aspects of cell biology, embryonic development, genetic control, birth defects and ethical issues.

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