Making Babies, Part 2
Several developmental biologists have suggested that a human is simply an egg’s way of making another egg. In this regard, the female reproductive system is the means by which an egg is made. It is also the location where the sperm fertilizes the egg, and where the fertilized egg divides, matures, and becomes an independent, living individual.
Ovaries
Ovaries are the organs for the storage, development, maturation, and eventual release of the egg. These paired organs are found in the lower abdominal quadrants on the right and left sides and are attached to the pelvis via ligaments.
Anatomy
The paired almond-shaped ovaries are divided into an outer cortex and inner medulla and are covered by a connective tissue capsule called the tunica albuginea.
Within the cortex, there is a connective tissue network of stromal cells and ovarian follicles in various stages of development. Each of the ovarian follicles contain an egg precursor (oogonia) as well as the supportive cells (follicular cells) that surround the future egg.
The medulla of the ovary is populated with larger maturing follicles and the remnants of follicles from previous cycles that are in various stages of degeneration and regression.
Follicles
Follicles are the basic units of the egg (oocyte) and supportive cells that surround the egg and occur in various developmental stages within the ovary.
The most abundant of the follicles and those found at the periphery of the cortex, adjacent to the capsule, are the primordial follicles. These are made up of a single layer of flattened follicular cells surrounding a primary oocyte (egg). As the smallest of the follicles (approximately 25 mm), the oocyte is paused in prophase I of meiosis, a process that continues when stimulated by hormones and leads to the maturation of the oocyte.
The next stage of follicular development is the unilaminar primary follicle. As the name implies, this is an oocyte that has grown much larger (approximately 100–150 mm), and is surrounded by a single layer of cube-shaped cells, which is the principal characteristic of this stage compared to other follicular stages. Throughout follicular development, these cells may often be called granulosa cells. Another difference between primordial and primary follicles is the formation of a molecular zone situated between the oocyte and the granulosa cells called the zona pellucida. This layer remains associated with the egg even after ovulation and presents a molecular barrier that the sperm must penetrate in order to fertilize the egg.
Still a primary follicle, but with multiple granulosa layers, a multilaminar primary follicle marks the next stage of follicle maturation.
The granulosa cells begin to produce a fluid that is secreted into the spaces between the cells and pool into fluid-filled spaces within the layer. These small pools come together as more fluid is produced until a single large cavity is produced called the antrum. This fluid, the liquor folliculi, contains hormones such as progesterone, and is the driving force behind the follicle’s expansion until it is the size of the entire ovary. This leads to a hydrostatic pressure that ruptures the follicle and the capsule of the ovary (ovulation).
As the antrum forms, a single layer of granulosa cells remains next to the zona pellucida. This is called the corona radiata. This too presents a fertilization barrier for sperm. Other granulosa cells (i.e., cumulus oophorus) surround the corona cells and also connect the egg to the surrounding stromal cells that have formed the outer compartment for the expanding follicle.
While several follicles are stimulated to begin their development, typically only a single follicle expands to the point of rupturing and releasing an egg into the female reproductive tract. As soon as the oocyte, zona pellucida, and corona radiata package detaches from the walls of the follicle and is free floating in the fluid, it is called a mature or Graafian follicle. While millions of eggs are in the ovaries, only approximately 500 will mature and be released (ovulated) during the reproductive life of the female. The remainder degenerate.
Female Reproductive Tract
The female reproductive tract facilitates three critical functions to ensure fertilization and continuation of the species. First, the released oocyte is transported inside of the tract where fertilization occurs. This necessitates the second important role, a pathway through which sperm must migrate. Lastly, the tract provides a safe haven where the newly created individual can develop until he or she can survive relatively independently.
Uterine Tubes
Also called the Fallopian tubes, this portion of the female reproductive tract guides the oocyte toward the uterus, allows sperm to migrate within the tube, and provides a location for fertilization to occur. The uterine tube is connected to the uterus, and extends laterally on the right and left side as the body of the tube (isthmus) expands near the ovary to form the ampulla. This expanded region is the location where successful fertilization must occur for implantation and pregnancy to occur. The opening of the tube, the infundibulum, is bounded by many fingerlike processes (fimbriae), which surround the ovary and aid in funneling the ovulated oocyte into the tract. Additionally, some of the cells that line the uterine tube have cilia on their surface that move and create a current toward the uterus, thus pulling the oocyte in the correct direction.
Uterus
The uterus is the expanded, muscular portion of the reproductive tract in which the developing individual is protected and nourished during pregnancy. Because of the thick muscular layers, this is also the engine that drives the delivery of the baby at parturition (childbirth).
Anatomy
Positioned between the right and left fallopian tubes along the midline of the pelvic region, the uterus is also located between the urinary bladder and the rectum. Initially around 50–60 grams and only about 3 inches long, the uterus can expand greatly to accommodate the growing baby during pregnancy. The top or superior part of the uterus is the fundus (much like that of the stomach in shape), the middle portion and the majority of the uterus is the body, and the narrowed neck of the inferior part of the uterus that is attached to the vagina is the cervix. Two layers make up the wall of the uterus: the inner endometrium and outer myometrium.
The inner endometrium layer facilitates implantation of the developing individual, initially nourishes the growing embryo, and assists in the formation of the placenta. Each month, this part of the uterine wall expands and becomes populated with spiraling blood vessels and endometrial glands in preparation for possible implantation of an embryo. However, if implantation does not occur within a defined amount of time (implantation window), the bulk of the endometrium (the surface functional layer) is shed during a menstrual period (menses). The base (basalis layer) of the endometrium remains and divides to regrow the thick endometrium for the next monthly cycle.
The myometrium is divided into three layers of smooth muscle that respond to hormonal signals near the end of pregnancy and spontaneously and rhythmically contract to deliver the baby. Signaled by the baby, the mother’s pituitary gland secretes oxytocin that leads to the initial contractions. These contractions lead to an increase in oxytocin production, which causes stronger and more frequent contractions. This cycle continues until the baby and then the placenta are delivered, at which time the contractions slow and stop.
What are the initial precursor contractions that precede labor called?
Braxton-Hicks contractions (practice contractions) are myometrial contractions caused by nonhormonal signals and are not associated with actual labor.
Vagina
The vagina is the terminal end of the female reproductive tract and functions as the female copulatory (sex) organ, as well as providing a pathway for the delivery of the baby. The male penis is inserted into this muscular tube during intercourse. Mucus glands and other secretory cells that are essential for the last developmental stage of sperm maturation are present in the wall of the vagina. Ejaculated sperm are incapable of fertilizing an egg. Capacitation (maturation) of the sperm occurs within the vagina, and results in the sperm’s ability to fertilize the egg. While the inferior end of the vagina is open, the superior end is bounded by the cervix, which contains a small opening through which the sperm must pass to get to the remainder of the tract.
External Genitalia
Made up of the same developmental building materials that shaped the male genitalia, the external genitalia of females remains open as fleshy folds of tissue that surround the opening of the vagina and urethra. Additionally, the phallic tissue that became the penile head (glans penis) in males is formed into the clitoris in females.
Labia
The two paired folds of tissue surrounding the opening of the vagina are the labia and collectively form the vulva. The outermost pair, which is thicker, fleshier, and usually covered in hair, is the labia majora. These are produced from the same tissue that was used to make the scrotum in males (labioscrotal swellings). Deeper and more medial are the labia minora. These are the thinner, more elongated folds that immediately surround the cavity which leads to the vagina and the urethra (vulval vestibule).
Clitoris
The phallic organ for females is the clitoris. The clitoris is a bundle of highly sensitive tissue located at the ventral junction of the labia (both minora and majora). It plays a major role in female arousal during intercourse and leads to orgasm and spasmodic contractions of the vaginal walls, which can assist in propelling sperm along the female reproductive tract.
Female Sex Hormones
These steroid hormones regulate both the menstrual cycle (egg production) and the estrus cycle (sexual desire) in females.
Estrogen
This is actually a group of steroid hormones that includes estradiol, which is the most abundant of the estrogens during the reproductive years of females. For women, estrogens lead to the expression of the secondary female characteristics, such as enlarged breasts, and also functions in the menstrual cycle to expand the endometrial layer.
Progesterone
Progesterone, the predominant progestogen in the female body, is also involved in similar functions as estrogens. This hormone, often called the hormone of pregnancy, affects the breasts and leads to milk production and lactation. It also plays a cyclical role along with estrogen in the menstrual cycle of females.
Reproductive Cycle
The monthly reproductive cycle of females is often called the menstrual cycle because, if fertilization and implantation does not occur, menses (shedding of the endometrium and bleeding) will proceed.
Follicle Maturation
The first half of the menstrual cycle, the follicular phase, begins with the development of several primordial follicles. This is triggered by an increase in the production of the pituitary hormone called follicle-stimulating hormone (FSH). The levels of FSH peak in the first few days of the cycle and slowly taper off until around the twelfth day when there is a spike of luteinizing hormone (LH), which triggers ovulation. At this point, estrogen has already begun rising and also peaks at ovulation before starting to decrease. Progesterone levels remain low during this phase.
While these hormones are directing follicular maturation, they also cause the endometrium to expand greatly. This is called the proliferative phase for the uterine lining.
Ovulation
As the liquor folliculi of the Graafian follicle increase beyond the capacity of the thecal cells and the ovarian capsule, the follicle and ovary rupture and the free-floating unit of oocyte and its supportive cells are expelled into the abdominal cavity. However, since the infundibulum of the uterine tube is nearby, the oocyte usually makes its way into the female reproductive tract easily. This final increase in volume and pressure is triggered by a spike in the level of LH from the pituitary gland. Also at this time, levels of progesterone begin to slowly increase, to peak in the middle of the next stage.
Luteal Phase
FSH and LH trigger the remaining thecal cells (lutein cells) of the follicular remnant to transform into a corpus luteum, a temporary structure capable of producing both progesterone and estrogen. This causes the pituitary to shut down both FSH and LH production. However, these hormones are essential to maintaining the function of the corpus luteum. If pregnancy does not occur, it is only a matter of time before the corpus luteum destructs. Then progesterone and estrogen levels drop, which is the trigger for menses.
This phase also corresponds to the secretory period for the uterus, in which the endometrium produces endometrial glandular secretions to nourish and support an implanted embryo if this should occur.
If pregnancy does occur, the developing embryo and precursor cells of the placenta produce hormones, which keep the corpus luteum active for a much longer period of time to prevent the loss of the endometrial lining (the embryo depends on this). The major hormone produced is human chorionic gonadotropin (HCG), which is the target for many early pregnancy tests.
What is menopause?
This phase marks the end of the reproductive cycle for females. It is characterized by the absence of menses (a.k.a. amenorrhea). This is often caused by slow changes in the hormone levels in the body, which leads to the failure of ovarian follicles’ ability to mature, ovulate, and shed the endometrial lining.