Catherine M. DeUgarte, MD
In the urogenital system, knowledge of the embryology is crucial in understanding the functions and interconnections between the reproductive and urologic systems. The adult genital and urinary systems are distinct in both function and anatomy, with the exception of the male urethra, where the 2 systems are interconnected. During development, these 2 systems are closely associated. The initial developmental overlap of these systems occurs 4–12 weeks after fertilization. The complexity of developmental events in these systems is evident by the incomplete separation of the 2 systems found in some congenital anomalies. For the sake of clarity, this chapter describes the embryology of each system separately, rather than following a strict developmental chronology.
In view of the complexity and duration of differentiation and development of the genital and urinary systems, it is not surprising that the incidence of malformations involving these systems is one of the highest (10%) of all body systems. Etiologies of congenital malformations are sometimes categorized on the basis of genetic, environmental, or genetic-plus-environmental (so-called polyfactorial inheritance) factors. Known genetic and inheritance factors reputedly account for about 20% of anomalies detected at birth, aberration of chromosomes for nearly 5%, and environmental factors for nearly 10%. The significance of these statistics must be viewed against reports that (1) an estimated one-third to one-half of human zygotes are lost during the first week of gestation and (2) the cause of possibly 70% of human anomalies is unknown. Even so, congenital malformations remain a matter of concern because they are detected in nearly 3% of infants, and 20% of perinatal deaths are purportedly due to congenital anomalies.
The inherent pattern of normal development of the genital system can be viewed as one directed toward somatic “femaleness,” unless development is directed by factors for “maleness.” The presence and expression of a Y chromosome (and its testis-determining genes) in a normal 46,XY karyotype of somatic cells directs differentiation toward a testis, and normal development of the testis makes available hormones for the selection and differentiation of the genital ducts. When male hormones are present, the mesonephric (wolffian) system persists; when male hormones are not present, the “female” paramesonephric (müllerian) ducts persist. Normal feminization or masculinization of the external genitalia is also a result of the respective timely absence or presence of androgen.
An infant usually is reared as female or male according to the appearance of the external genitalia. However, genital sex is not always immediately discernible, and the choice of sex of rearing can be an anxiety-provoking consideration. Unfortunately, even when genital sex is apparent, later clinical presentation may unmask disorders of sexual differentiation that can lead to problems in psychological adjustment. Whether a somatic disorder is detected at birth or later, investigative backtracking through the developmental process is necessary for proper diagnosis and treatment.
Overview of the First 4 Weeks of Development*
Transformation of the bilaminar embryonic disk into a trilaminar disk composed of ectoderm, mesoderm, and endoderm (the 3 embryonic germ layers) occurs during the third week by a process called gastrulation (Fig. 2–1). During this process, a specialized thickening of epiblast, the primitive streak, elongates through the midline of the disk. Some epiblastic cells become mesoblastic cells, which migrate peripherally between the epiblast and hypoblast, forming a middle layer of embryonic mesoderm. Other mesoblastic cells migrate into the hypoblastic layer and form embryonic endoderm, which displaces the hypoblastic cells. The remaining overlying epiblast becomes the embryonic ectoderm.
Figure 2–1. Schematic overview of embryonic development of progenitory urinary and genital tissues and structures considered to be derivatives of embryonic ectoderm, mesoderm, or endoderm. Numbers indicate the weeks after fertilization when the indicated developmental change occurs. GI, gastrointestinal.
By the end of the third week, 3 clusters of mesoderm are organized on both sides of the midline neural tube. From medial to lateral, these clusters are paraxial mesoderm, which forms much of the axial skeleton; intermediate mesoderm, which is the origin of the urogenital ridge and, hence, much of the reproductive and excretory systems (Fig. 2–2); and lateral plate mesoderm, which splits and takes part in body cavity formation. The intermediate mesoderm is located between the paraxial and lateral plate mesoderm and is the origin of the urogenital ridge and, hence, much of the reproductive and excretory systems (Fig. 2–2). The primitive streak regresses after the fourth week. Rarely, degeneration of the streak is incomplete, and presumptive remnants form a teratoma in the sacrococcygeal region of the fetus (more common in females than in males).
Figure 2–2. Schematic drawing of formation of the definitive kidney and its collecting ducts. The pronephric duct is probably the only structure that participates in all 3 urinary systems, as its caudal portion continues to grow and is called the mesonephric duct when the mesonephric system develops. (Explanatory symbols are given in Fig. 2–1.)
Weeks 4 through 8 of development are called the embryonic period (the fetal period is from week 9 to term) because formation of all major internal and external structures, including the 2 primary forerunners of the urogenital system (urogenital ridge and urogenital sinus), begins during this time. During this period the embryo is most likely to develop major congenital or acquired morphologic anomalies in response to the effects of various agents. During the fourth week, the shape of the embryo changes from that of a trilaminar disk to that of a crescentic cylinder. The change results from “folding,” or flexion, of the embryonic disk in a ventral direction through both its transverse and longitudinal planes. Flexion occurs as midline structures (neural tube and somites) develop and grow at a faster pace than more lateral tissues (ectoderm, 2 layers of lateral plate mesoderm enclosing the coelom between them, and endoderm). Thus, during transverse folding, the lateral tissues on each side of the embryo curl ventromedially and join the respective tissues from the other side, creating a midline ventral tube (the endoderm-lined primitive gut), a mesoderm-lined coelomic cavity (the primitive abdominopelvic cavity), and the incomplete ventral and lateral body wall. Concurrent longitudinal flexion ventrally of the caudal region of the disk establishes the pouchlike distal end, or cloaca, of the primitive gut as well as the distal attachment of the cloaca to the yolk sac through the allantois of the sac (Fig. 2–3).
Figure 2–3. Early stage in the formation of the mesonephric kidneys and their collecting ducts in the urogenital ridge. The central tissue of the ridge is the nephrogenic cord, in which the mesonephric tubules are forming. The mesonephric ducts grow toward (arrows) and will open into the cloaca. About 5 weeks’ gestation.
A noteworthy point (see The Gonads) is that the primordial germ cells of the later-developing gonad initially are found close to the allantois and later migrate to the gonadal primordia. Subsequent partitioning of the cloaca during the sixth week results in formation of the anorectal canal and the urogenital sinus, the progenitor of the urinary bladder, urethra, vagina, and other genital structures (Fig. 2–1and Table 2–1; see Subdivision of the Cloaca & Formation of the Urogenital Sinus).
Table 2–1. Adult derivatives and vestigial remains of embryonic urogenital structures.
Embryonic folding also moves the intermediate mesoderm—the forerunner of the urogenital ridge—to its characteristic developmental locations as bilateral longitudinal bulges in the dorsal wall of the new body cavity and lateral to the dorsal mesentery of the new gut tube. By the end of the fourth week of development, the principal structures (urogenital ridge and cloaca) and tissues that give rise to the urogenital system are present.
Tables 2–1 and 2–2 provide a general overview of urogenital development.
Table 2–2. Developmental chronology of the human urogenital system.
THE URINARY SYSTEM
Three excretory “systems” form successively, with temporal overlap, during the embryonic period. Each system has a different excretory “organ,” but the 3 systems share anatomic continuity through development of their excretory ducts. The 3 systems are mesodermal derivatives of the urogenital ridge (Figs. 2–2 and 2–3), part of which becomes a longitudinal mass, the nephrogenic cord. The pronephros, or organ of the first system, exists rudimentarily, is nonfunctional, and regresses during the fourth week. However, the developing pronephric ducts continue to grow and become the mesonephric ducts of the subsequent kidney, the mesonephros. The paired mesonephroi exist during 4–8 weeks as simplified morphologic versions of the third, or permanent, set of kidneys, and they may have transient excretory function. Although the mesonephroi degenerate, some of their tubules, called epigenital mesonephric tubules, persist to participate in formation of the gonad and male ductuli efferentes (Fig. 2–4). The permanent kidney, the metanephros, begins to form in response to an inductive influence of a diverticulum of the mesonephric ducts during the fifth week and becomes functional at 10–13 weeks.
Figure 2–4. Schematic drawing of the formation of the gonads and genital ducts.
Differentiation of the caudal segment of the mesonephric ducts results in (1) incorporation of part of the ducts into the wall of the urogenital sinus (early vesicular trigone, see following text), and (2) formation of a ductal diverticulum, which plays an essential role in formation of the definitive kidney. If male sex differentiation occurs, the major portion of each duct becomes the epididymis, ductus deferens, and ejaculatory duct. Only small vestigial remnants of the duct sometimes persist in the female (Gartner’s duct; duct of the epoophoron).
Metanephros (Definitive Kidney)
A. Collecting Ducts
By the end of the fifth week, a ureteric bud, or metanephric diverticulum, forms on the caudal part of the mesonephric duct close to the cloaca. The bud gives rise to the collecting tubules, calices, renal pelvis, and ureter (Fig. 2–2). The stalk of the elongating bud will become the ureter when the ductal segment between the stalk and the cloaca becomes incorporated into the wall of the urinary bladder (which is a derivative of the partitioned cloaca, see text that follows; Figs. 2–5 to 2–8). The expanded tip, or ampulla, of the bud grows into the adjacent metanephric mesoderm (blastema) and subdivides successively into 12–15 generations of buds, or eventual collecting tubules. From weeks 10–14, dilatation of the early generations of tubular branches successively produces the renal pelvis, the major calices, and the minor calices, while the middle generations form the medullary collecting tubules. The last several generations of collecting tubules grow centrifugally into the cortical region of the kidney between weeks 24 and 36.
Figure 2–5. Left-side view of urogenital system and cloacal region prior to subdivision of cloaca by urorectal septum (Tourneux and Rathke folds). Position of future paramesonephric duct is shown (begins in the sixth week). Gonad is in the indifferent stage (sexually undifferentiated).
Figure 2–6. Left-side view of urogenital system. Urorectal septum nearly subdivides the cloaca into the urogenital sinus and the anorectal canal. Paramesonephric ducts do not reach the sinus until the ninth week. Gonad is sexually undifferentiated. Note incorporation of caudal segment of mesonephric duct into urogenital sinus (compare with Fig. 2–5).
Figure 2–7. Left-side view of urogenital system at an early stage of male sexual differentiation. Phallic part of urogenital sinus is proliferating anteriorly to form the urethral plate and groove. Seminal vesicles and prostatic buds are shown at a more advanced stage (about 12 weeks) for emphasis.
Figure 2–8. Left-side view of urogenital system at an early stage of female sexual differentiation. Paramesonephric (müllerian) ducts have fused caudally (to form uterovaginal primordium) and contacted the pelvic part of the urogenital sinus.
B. Nephrons
Continued maintenance of the intimate relationship of the metanephric blastema and ampulla is necessary for normal formation of the definitive excretory units (nephrons), which starts at about the eighth week. Formation of urine purportedly begins at about weeks 10–13, when an estimated 20% of the nephrons are morphologically mature.
The last month of gestation is marked by interstitial growth, hypertrophy of existing components of uriniferous tubules, and the disappearance of bud primordia for collecting tubules. Opinions differ about whether formation of nephrons ceases prenatally at about 28 or 32 weeks or postnatally during the first several months. If the ureteric bud fails to form, undergoes early degeneration, or fails to grow into the nephrogenic mesoderm, aberrations of nephrogenesis result. These may be nonthreatening (unilateral renal agenesis), severe, or even fatal (bilateral renal agenesis, polycystic kidney).
C. Positional Changes
Figure 2–9 illustrates relocation of the kidney to a deeper position within the posterior body wall, as well as the approximately 90-degree medial rotation of the organ on its longitudinal axis. Rotation and lateral positioning probably are facilitated by the growth of midline structures (axial skeleton and muscles). The “ascent” of the kidney between weeks 5 and 8 can be attributed largely to differential longitudinal growth of the rest of the lumbosacral area and to the reduction of the rather sharp curvature of the caudal region of the embryo. Some migration of the kidney may also occur. Straightening of the curvature also may be attributable to relative changes in growth, especially the development of the infraumbilical abdominal wall. As the kidney moves into its final position (lumbar 1–3 by the 12th week), its arterial supply shifts to successively higher aortic levels. Ectopic kidneys can result from abnormal “ascent.” During the seventh week, the “ascending” metanephroi closely approach each other near the aortic bifurcation. The close approximation of the 2 developing kidneys can lead to fusion of the lower poles of the kidneys, resulting in formation of a single horseshoe kidney, the ascent of which would be arrested by the stem of the interior mesenteric artery. Infrequently, a pelvic kidney results from trapping of the organ beneath the umbilical artery, which restricts passage out of the pelvis.
Figure 2–9. Positional changes of the definitive kidney at 5 different stages but projected on one cross-sectional plane.
THE GENITAL SYSTEM
Sexual differentiation of the genital system occurs in a basically sequential order: genetic, gonadal, ductal, and genital. Genetic sex is determined at fertilization by the complement of sex chromosomes (ie, XY specifies a genotypic male and XX a female). However, early morphologic indications of the sex of the developing embryo do not appear until about the eighth or ninth week after conception. Thus, there is a so-called indifferent stage, when morphologic identity of sex is not clear or when preferential differentiation for one sex has not been imposed on the sexless primordia. This is characteristic of early developmental stages for the gonads, genital ducts, and external genitalia. When the influence of genetic sex has been expressed on the indifferent gonad, gonadal sex is established. The SRY (sex-determining region of the Y chromosome) gene in the short arm of the Y chromosome of normal genetic males is considered the best candidate for the gene encoding for the testis-determining factor (TDF). TDF initiates a chain of events that results in differentiation of the gonad into a testis with its subsequent production of antimüllerian hormone and testosterone, which influences development of somatic “maleness” (see Testis). Normal genetic females do not have the SRY gene, and the early undifferentiated medullary region of their presumptive gonad does not produce the TDF (see Ovary).
The testis and ovary are derived from the same primordial tissue, but histologically visible differentiation toward a testis occurs sooner than that toward an ovary. An “ovary” is first recognized by the absence of testicular histogenesis (eg, thick tunica albuginea) or by the presence of germ cells entering meiotic prophase between the 8th and about the 11th week. The different primordia for male and female genital ducts exist in each embryo during overlapping periods, but establishment of male or female ductal sex depends on the presence or absence, respectively, of testicular products and the sensitivity of tissues to these products. The 2 primary testicular products are androgenic steroids (testosterone and nonsteroidal antimüllerian hormone) (see Testis). Stimulation by testosterone influences the persistence and differentiation of the “male” mesonephric ducts (wolffian ducts), whereas antimüllerian hormone influences regression of the “female” paramesonephric ducts (müllerian ducts). Absence of these hormones in a nonaberrant condition specifies persistence of müllerian ducts and regression of wolffian ducts (ie, initiation of development of the uterus and uterine tubes). Genital sex (external genitalia) subsequently develops according to the absence or presence of androgen. Thus, the inherent pattern of differentiation of the genital system can be viewed as one directed toward somatic “femaleness” unless the system is dominated by certain factors for “maleness” (eg, gene expression of the Y chromosome, androgenic steroids, and antimüllerian hormone).
THE GONADS
Indifferent (Sexless) Stage
Gonadogenesis temporally overlaps metanephrogenesis and interacts with tissues of the mesonephric system. Formation of the gonad is summarized schematically in Figure 2–4. Around the fifth week, the midportion of each urogenital ridge thickens as cellular condensation forms the gonadal ridge. For the next 2 weeks, this ridge is an undifferentiated cell mass, lacking either testicular or ovarian morphology. As shown in Figure 2–4, the cell mass consists of (1) primordial germ cells, which translocate into the ridge, and a mixture of somatic cells derived by (2) proliferation of the coelomic epithelial cells, (3) condensation of the underlying mesenchyme of part of the urogenital ridge, and (4) ingrowth of mesonephric-derived cells.
The end of the gonadal indifferent stage in the male is near the middle of the seventh week, when a basal lamina delineates the coelomic epithelium and the developing tunica albuginea separates the coelomic epithelium from the developing testicular cords. The indifferent stage in the female ends around the ninth week, when the first oogonia enter meiotic prophase.
Primordial germ cells, presumptive progenitors of the gametes, become evident in the late third to early fourth week in the dorsocaudal wall of the yolk sac and the mesenchyme around the allantois. The allantois is a caudal diverticulum of the yolk sac that extends distally into the primitive umbilical stalk and, after embryonic flexion, is adjacent proximally to the cloacal hindgut. The primordial germ cells are translocated from the allantoic region (about the middle of the fourth week) to the urogenital ridge (between the middle of the fifth week and late in the sixth week). It is not known whether primordial germ cells must be present in the gonadal ridge for full differentiation of the gonad to occur. The initial stages of somatic development appear to occur independently of the germ cells. Later endocrine activity in the testis, but not in the ovary, is known to occur in the absence of germ cells. The germ cells appear to have some influence on gonadal differentiation at certain stages of development.
Testis
During early differentiation of the testis, there are condensations of germ cells and somatic cells, which have been described as platelike groups, or sheets. These groups are at first distributed throughout the gonad and then become more organized as primitive testicular cords. The cords begin to form centrally and are arranged somewhat perpendicular to the long axis of the gonad. In response to TDF, these cords differentiate into Sertoli cells. The first characteristic feature of male gonadal sex differentiation is evident around week 8, when the tunica albuginea begins to form in the mesenchymal tissue underlying the coelomic epithelium. Eventually, this thickened layer of tissue causes the developing testicular cords to be separated from the surface epithelium and placed deeper in the central region of the gonad. The surface epithelium reforms a basal lamina and later thins to a mesothelial covering of the gonad. The testicular cords coil peripherally and thicken as their cellular organization becomes more distinct. A basal lamina eventually develops in the testicular cords, although it is not known if the somatic cells, germ cells, or both are primary contributors to the lamina.
Throughout gonadal differentiation, the developing testicular cords appear to maintain a close relationship to the basal area of the mesonephric-derived cell mass. An interconnected network of cords, rete cords, develops in this cell mass and gives rise to the rete testis. The rete testis joins centrally with neighboring epigenital mesonephric tubules, which become the efferent ductules linking the rete testis with the epididymis, a derivative of the mesonephric duct. With gradual enlargement of the testis and regression of the mesonephros, a cleft forms between the 2 organs, slowly creating the mesentery of the testis, the mesorchium.
The differentiating testicular cords are made up of primordial germ cells (primitive spermatogonia) and somatic “supporting” cells (sustentacular cells, or Sertoli cells). Some precocious meiotic activity has been observed in the fetal testis. Meiosis in the germ cells usually does not begin until puberty; the cause of this delay is unknown. Besides serving as “supporting cells” for the primitive spermatogonia, Sertoli cells also produce the glycoprotein antimüllerian hormone (also called müllerian-inhibiting substance). Antimüllerian hormone causes regression of the paramesonephric (müllerian) ducts, apparently during a very discrete period of ductal sensitivity in male fetuses. At puberty, the seminiferous cords mature to become the seminiferous tubules, and the Sertoli cells and spermatogonia mature.
Shortly after the testicular cords form, the steroid-producing interstitial (Leydig) cells of the extracordal compartment of the testis differentiate from stromal mesenchymal cells, probably due to antimüllerian hormone. Mesonephric-derived cells may also be a primordial source of Leydig cells. Steroidogenic activity of Leydig cells begins near the 10th week. High levels of testosterone are produced during the period of differentiation of external genitalia (weeks 11–12) and maintained through weeks 16–18. Steroid levels then rise or fall somewhat in accordance with changes in the concentration of Leydig cells. Both the number of cells and the levels of testosterone decrease around the fifth month.
Ovary
A. Development
In the normal absence of the Y chromosome or the sex-determining region of the Y chromosome (SRY gene; see The Genital System), the somatic sex cords of the indifferent gonad do not produce TDF. In the absence of TDF, differentiation of the gonad into a testis and its subsequent production of antimüllerian hormone and testosterone do not occur (see Testis). The indifferent gonad becomes an ovary. Complete ovarian differentiation seems to require two X chromosomes (XO females exhibit ovarian dysgenesis, in which ovaries have precociously degenerated germ cells and no follicles and are present as gonadal “streaks”). The first recognition of a developing ovary around weeks 9–10 is based on the temporal absence of testicular-associated features (most prominently, the tunica albuginea) and on the presence of early meiotic activity in the germ cells.
Early differentiation toward an ovary involves mesonephric-derived cells “invading” the basal region (adjacent to mesonephros) and central region of the gonad (central and basal regions represent the primitive “medullary” region of the gonad). At the same time, clusters of germ cells are displaced somewhat peripherally into the “cortical” region of the gonad. Some of the central mesonephric cells give rise to the rete system that subsequently forms a network of cords (intraovarian rete cords) extending to the primitive cortical area. As these cords extend peripherally between germ clusters, several epithelial cell proliferations extend centrally, and some mixing of these somatic cells apparently takes place around the germ cell clusters. These early cordlike structures are more irregularly distributed than early cords in the testis and not distinctly outlined. The cords open into clusters of germ cells, but all germ cells are not confined to cords. The first oogonia that begin meiosis are located in the innermost part of the cortex and are the first germ cells to contact the intraovarian rete cords.
Folliculogenesis begins in the innermost part of the cortex when the central somatic cells of the cord contact and surround the germ cells while an intact basal lamina is laid down. These somatic cells are morphologically similar to the mesonephric cells that form the intraovarian rete cords associated with the oocytes and apparently differentiate into the presumptive granulosa cells of the early follicle. Folliculogenesis continues peripherally. Between weeks 12 and 20 of gestation, proliferative activity causes the surface epithelium to become a thickened, irregular multilayer of cells. In the absence of a basal lamina, the cells and apparent epithelial cell cords mix with underlying tissues. These latter cortical cords often retain a connection to and appear similar to the surface epithelium. The epithelial cells of these cords probably differentiate into granulosa cells and contribute to folliculogenesis, although this occurs after the process is well under way in the central region of the gonad. Follicles fail to form in the absence of oocytes or with precocious loss of germ cells, and oocytes not encompassed by follicular cells degenerate.
Stromal mesenchymal cells, connective tissue, and somatic cells of cords not participating in folliculogenesis form the ovarian medulla in the late fetal ovary. Individual primordial follicles containing diplotene oocytes populate the inner and outer cortex of this ovary. The rete ovarii may persist, along with a few vestiges of mesonephric tubules, as the vestigial epoophoron near the adult ovary. Finally, similar to the testicular mesorchium, the mesovariumeventually forms as a gonadal mesentery between the ovary and old urogenital ridge. Postnatally, the epithelial surface of the ovary consists of a single layer of cells continuous with peritoneal mesothelium at the ovarian hilum. A thin, fibrous connective tissue, the tunica albuginea, forms beneath the surface epithelium and separates it from the cortical follicles.
B. Anomalies of the Ovaries
Anomalies of the ovaries encompass a broad range of developmental errors from complete absence of the ovaries to supernumerary ovaries. The many variations of gonadal disorders usually are subcategorized within classifications of disorders of sex determination. Unfortunately, there is little consensus for a major classification, although most include pathogenetic consideration. Extensive summaries of the different classifications are offered in the references to this chapter.
Congenital absence of the ovary (no gonadal remnants found) is very rare. Two types have been considered, agenesis and agonadism. By definition, agenesis implies that the primordial gonad did not form in the urogenital ridge, whereas agonadism indicates the absence of gonads that may have formed initially and subsequently degenerated. It can be difficult to distinguish one type from the other on a practical basis. For example, a patient with female genital ducts and external genitalia and a 46,XY karyotype could represent either gonadal agenesis or agonadism. In the latter condition, the gonad may form but undergo early degeneration and resorption before any virilizing expression is made. Whenever congenital absence of the ovaries is suspected, careful examination of the karyotype, the external genitalia, and the genital ducts must be performed.
Descriptions of agonadism usually have indicated that the external genitalia are abnormal (variable degree of fusion of labioscrotal swellings) and that either very rudimentary ductal derivatives are present or there are no genital ducts. The cause of agonadism is unknown, although several explanations have been suggested, such as (1) failure of the primordial gonad to form, along with abnormal formation of ductal anlagen, and (2) partial differentiation and then regression and absorption of testes (accounting for suppression of müllerian ducts but lack of stimulation of mesonephric, or wolffian, ducts). Explanations that include teratogenic effects or genetic defects are more likely candidates in view of the associated incidence of nonsexual somatic anomalies with the disorder. The streak gonad is a product of primordial gonadal formation and subsequent failure of differentiation, which can occur at various stages. The gonad usually appears as a fibrouslike cord of mixed elements (lacking germ cells) located parallel to a uterine tube. Streak gonads are characteristic of gonadal dysgenesis and a 45,XO karyotype (Turner’s syndrome;distinctions are drawn between Turner’s syndrome and Turner’s stigmata when consideration is given to the various associated somatic anomalies of gonadal dysgenesis). However, streak gonads may be consequent to genetic mutation or hereditary disease other than the anomalous karyotype.
Ectopic ovarian tissue occasionally can be found as accessory ovarian tissue or as supernumerary ovaries. The former may be a product of disaggregation of the embryonic ovary, and the latter may arise from the urogenital ridge as independent primordia.
SUBDIVISION OF THE CLOACA & FORMATION OF THE UROGENITAL SINUS
The endodermally lined urogenital sinus is derived by partitioning of the endodermal cloaca. It is the precursor of the urinary bladder in both sexes and the urinary and genital structures specific to each sex (Fig. 2–1). The cloaca is a pouchlike enlargement of the caudal end of the hindgut and is formed by the process of “folding” of the caudal region of the embryonic disk between 4 and 5 weeks’ gestation (see Overview of the First 4 Weeks of Development; Figs. 2–1 and 2–3). During the “tail-fold” process, the posteriorly placed allantois, or allantoic diverticulum of the yolk sac, becomes an anterior extension of the cloaca (Figs. 2–3 and 2–5). Soon after the cloaca forms, it receives posterolaterally the caudal ends of the paired mesonephric ducts and hence becomes a junctional cistern for the allantois, the hindgut, and the ducts. A cloacal membrane, composed of ectoderm and endoderm, is the caudal limit of the primitive gut and temporarily separates the cloacal cavity from the extraembryonic confines of the amniotic cavity (Fig. 2–5).
Between weeks 5 and 7, 3 wedges of splanchnic mesoderm, collectively called the urorectal septum, proliferate in the coronal plane in the caudal region of the embryo to eventually subdivide the cloaca (Figs. 2–5 to 2–8). The superior wedge, called the Tourneux fold, is in the angle between the allantois and the primitive hindgut, and it proliferates caudally into the superior end of the cloaca (Fig. 2–5). The other 2 mesodermal wedges, called the Rathke folds, proliferate in the right and left walls of the cloaca. Beginning adjacent to the cloacal membrane, these laterally placed folds grow toward each other and the Tourneux fold. With fusion of the 3 folds creating a urorectal septum, the once single chamber is subdivided into the primitive urogenital sinus (ventrally) and the anorectal canal of the hindgut (dorsally; Figs. 2–6 to 2–8). The mesonephric ducts and allantois then open into the sinus. The uterovaginal primordium of the fused paramesonephric ducts will contact the sinusal wall between the mesonephric ducts early in the ninth week of development. However, it can be noted that the junctional point of fusion of the cloacal membrane and urorectal septum forms the primitive perineum (later differentiation creates the so-called perineal body of tissue) and subdivides the cloacal membrane into the urogenital membrane (anteriorly) and the anal membrane (posteriorly; Figs. 2–5, 2–8, and 2–10; see also Fig. 2–20).
THE GENITAL DUCTS
Indifferent (Sexless) Stage
Two pairs of genital ducts are initially present in both sexes: (1) the mesonephric (wolffian) ducts, which give rise to the male ducts and a derivative, the seminal vesicles; and (2) the paramesonephric (müllerian) ducts, which form the oviducts, uterus, and part of the vagina. When the adult structures are described as derivatives of embryonic ducts, this refers to the epithelial lining of the structures. Muscle and connective tissues of the differentiating structures originate from splanchnic mesoderm and mesenchyme adjacent to ducts. Mesonephric ducts are originally the excretory ducts of the mesonephric “kidneys” (see previous text), and they develop early in the embryonic period, about 2 weeks before development of paramesonephric ducts (weeks 6–10). The 2 pairs of genital ducts share a close anatomic relationship in their bilateral course through the urogenital ridge. At their caudal limit, both sets contact the part of the cloaca that is later separated as the urogenital sinus (Figs. 2–5, 2–6, and 2–10). Determination of the ductal sex of the embryo (ie, which pair of ducts will continue differentiation rather than undergo regression) is established initially by the gonadal sex and later by the continuing influence of hormones.
Figure 2–10. Diagrammatic comparison between male and female differentiation of internal genitalia.
Formation of each paramesonephric duct begins early in the sixth week as an invagination of coelomic epithelium in the lateral wall of the cranial end of the urogenital ridge and adjacent to each mesonephric duct. The free edges of the invaginated epithelium join to form the duct except at the site of origin, which persists as a funnel-shaped opening, the future ostium of the oviduct. At first, each paramesonephric duct grows caudally through the mesenchyme of the urogenital ridge and laterally parallel to a mesonephric duct. More inferiorly, the paramesonephric duct has a caudomedial course, passing ventral to the mesonephric duct. As it follows the ventromedial bend of the caudal portion of the urogenital ridge, the paramesonephric duct then lies medial to the mesonephric duct, and its caudal tip lies in close apposition to its counterpart from the opposite side (Fig. 2–10). At approximately the eighth week, the caudal segments of the right and left ducts fuse medially and their lumens coalesce to form a single cavity. This conjoined portion of the Y-shaped paramesonephric ducts becomes the uterovaginal primordium, or canal.
Male: Genital Ducts
A. Mesonephric Ducts
The mesonephric ducts persist in the male and, under the stimulatory influence of testosterone, differentiate into the internal genital ducts (epididymis, ductus deferens, and ejaculatory ducts). Near the cranial end of the duct, some of the mesonephric tubules (epigenital mesonephric tubules) of the mesonephric kidney persist lateral to the developing testis. These tubules form a connecting link, the ductuli efferentes, between the duct and the rete testis (Fig. 2–10). The cranial portion of each duct becomes the convoluted ductus epididymis. The ductus deferens forms when smooth muscle from adjacent splanchnic mesoderm is added to the central segment of the mesonephric duct. The seminal vesicle develops as a lateral bud from each mesonephric duct just distal to the junction of the duct and the urogenital sinus (Fig. 2–7). The terminal segment of the duct between the sinus and seminal vesicle forms the ejaculatory duct, which becomes encased by the developing prostate gland early in the 12th week (see Differentiation of the Urogenital Sinus). A vestigial remnant of the duct may persist cranially near the head of the epididymis as the appendix epididymis, whereas remnants of mesonephric tubules near the inferior pole of the testis and tail of the epididymis may persist as the paradidymis (Fig. 2–10).
B. Paramesonephric Ducts
The paramesonephric ducts begin to undergo morphologic regression centrally (and progress cranially and caudally) about the time they meet the urogenital sinus caudally (approximately the start of the ninth week). Regression is effected by nonsteroidal antimüllerian hormone produced by the differentiating Sertoli cells slightly before androgen is produced by the Leydig cells (see Testis). Antimüllerian hormone is produced from the time of early testicular differentiation until birth (ie, not only during the period of regression of the paramesonephric duct). However, ductal sensitivity to antimüllerian hormone in the male seems to exist for only a short “critical” time preceding the first signs of ductal regression. Vestigial remnants of the cranial end of the ducts may persist as the appendix testis on the superior pole of the testis (Fig. 2–10). Caudally, a ductal remnant is considered to be part of the prostatic utricle of the seminal colliculus in the prostatic urethra.
C. Relocation of the Testes & Ducts
Around weeks 5–6, a bandlike condensation of mesenchymal tissue in the urogenital ridge forms near the caudal end of the mesonephros. Distally, this gubernacular precursor tissue grows into the area of the undifferentiated tissue of the anterior abdominal wall and toward the genital swellings. Proximally, the gubernaculum contacts the mesonephric duct when the mesonephros regresses and the gonad begins to form. By the start of the fetal period, the mesonephric duct begins differentiation and the gubernaculum adheres indirectly to the testis via the duct, which lies in the mesorchium of the testis. The external genitalia differentiate over the seventh to about the 19th week. By the 12th week, the testis is near the deep inguinal ring, and the gubernaculum is virtually at the inferior pole of the testis, proximally, and in the mesenchyme of the scrotal swellings, distally.
Although the testis in early development is near the last thoracic segment, it is still close to the area of the developing deep inguinal ring. With rapid growth of the lumbar region and “ascent” of the metanephric kidney, the testis remains relatively immobilized by the gubernaculum, although there is the appearance of a lengthy transabdominal “descent” from an upper abdominal position. The testis descends through the inguinal canal around the 28th week and into the scrotum about the 32nd week. Testicular blood vessels form when the testis is located on the dorsal body wall and retain their origin during the transabdominal and pelvic descent of the testis. The mesonephric duct follows the descent of the testis and hence passes anterior to the ureter, which follows the retroperitoneal ascent of the kidney (Fig. 2–10).
Hutson JM, Balic A, Nation T, Southwell B. Cryptorchidism. Semin Pediatr Surg 2010;19:215–224. PMID: 20610195.
Shaw CM, Stanczyk FZ, Egleston BL, et al. Serum antimüllerian hormone in healthy premenopausal women. Fertil Steril 2011;95:2718–2721. PMID: 21704216.
Turner ME, Ely D, Prokop J, Milsted A. Sry, more than testis determination? Am J Physiol Regul Integr Comp Physiol 2011;301:R561–R571. PMID: 21677270.
Vallerie AM, Breech LL. Update in Müllerian anomalies: diagnosis, management, and outcomes Curr Opin Obstet Gynecol 2010;22:381–387. PMID: 20724925.
Female: Uterus & Uterine Tubes
A. Mesonephric Ducts
Virtually all portions of these paired ducts degenerate in the female embryo, with the exception of the most caudal segment between the ureteric bud and the cloaca, which is later incorporated into the posterior wall of the urogenital sinus (Figs. 2–5 and 2–6) as the trigone of the urinary bladder. Regression begins just after gonadal sex differentiation and is finished near the onset of the third trimester. Cystlike or tubular vestiges of mesonephric duct (Fig. 2–11) may persist to variable degrees parallel with the vagina and uterus (Gartner’s cysts). Other mesonephric remnants of the duct or tubules may persist in the broad ligament (epoophoron).
Figure 2–11. Female genital tract. Gubernacular derivatives and mesonephric vestiges are shown.
B. Paramesonephric Ducts
Differentiation of müllerian ducts in female embryos produces the uterine tubes, uterus, and probably the fibromuscular wall of the vagina. In contrast to the ductal/gonadal relationship in the male, ductal differentiation in the female does not require the presence of ovaries. Formation of the bilateral paramesonephric ducts during the second half of the embryonic period has been described [see Indifferent (Sexless) Stage]. By the onset of the fetal period, the 2 ducts are joined caudally in the midline, and the fused segment of the new Y-shaped ductal structure is the uterovaginal primordium (Fig. 2–8). The nonfused cranial part of each paramesonephric duct gives rise to the uterine tubes (oviducts), and the distal end of this segment remains open and will form the ostium of the oviduct.
Early in the ninth week, the uterovaginal primordium contacts medianly the dorsal wall of the urogenital sinus. This places the primordium at a median position between the bilateral openings of the mesonephric ducts, which joined the dorsal wall during the fifth week before subdivision of the urogenital sinus from the cloaca occurred (Figs. 2–8 and 2–9). A ventral protrusion of the dorsal wall of the urogenital sinus forms at the area of contact of the uterovaginal primordium with the wall and between the openings of the mesonephric ducts. In reference to its location, this protrusion is called the sinusal tubercle (sinus tubercle, paramesonephric tubercle, müllerian tubercle). This tubercle may consist of several types of epithelia derived from the different ducts as well as from the wall of the sinus.
Shortly after the sinusal tubercle forms, midline fusion of the middle and caudal portions of the paramesonephric ducts is complete, and the vertical septum (apposed walls of the fused ducts) within the newly established uterovaginal primordium degenerates, creating a single cavity or canal (Fig. 2–12). The solid tip of this primordium continues to grow caudally, while a mesenchymal thickening gradually surrounds the cervical region of the uterovaginal primordium. The primordium gives rise to the fundus, body, and isthmus of the uterus, specifically the endometrial epithelium and glands of the uterus. The endometrial stroma and smooth muscle of the myometrium are derived from adjacent splanchnic mesenchyme. The epithelium of the cervix forms from the lower aspect of the primordium. Development of the various components of the uterus covers the 3 trimesters of gestation. The basic structure is generated during the latter part of the first trimester. The initial formation of glands and muscular layer occurs near midgestation, whereas mucinous cells in the cervix appear during the third trimester.
Figure 2–12. Sagittal cutaway view of female urogenital sinus and uterovaginal primordium (fused paramesonephric ducts). Sinovaginal bulbs form in the 10th week.
The formation of the vagina is discussed in Differentiation of the Urogenital Sinus, even though the question of whether the vaginal epithelium is a sinusal or paramesonephric derivative (or both) has not been resolved. The fibromuscular wall of the vagina is generally considered to be derived from the uterovaginal primordium (Fig. 2–13).
Figure 2–13. Schematic drawing of differentiation of urogenital sinus and paramesonephric ducts in the female; formation of urinary bladder, urethra, uterine tubes, uterus, and vagina. (Explanatory symbols are given in Fig. 2–1.)
C. Relocation of the Ovaries & Formation of Ligaments
Transabdominal “descent” of the ovary, unlike that of the testis, is restricted to a relatively short distance, presumably (at least partly) because of attachment of the gubernaculum to the paramesonephric duct. Hence, relocation of the ovary appears to involve both (1) a passive rotatory movement of the ovary as its mesentery is drawn by the twist of the developing ductal mesenteries and (2) extensive growth of the lumbosacral region of the fetus. The ovarian vessels (like the testicular vessels) originate or drain near the point of development of the gonad, the arteries from the aorta just inferior to the renal arteries and the veins to the left renal vein or to the vena cava from the right gonad.
Initial positioning of the ovary on the anteromedial aspect of the urogenital ridge is depicted in Figure 2–10, as is the relationship of the paramesonephric duct lateral to the degenerating mesonephros, the ovary, and the urogenital mesentery. The urogenital mesentery between the ridge and the dorsal body wall represents the first mesenteric support for structures developing in the ridge.
Alterations within the urogenital ridge eventually result in formation of contiguous double-layered mesenteries supporting the ovary and segments of the paramesonephric ducts. Enlargement of the ovary and degeneration of the adjacent mesonephric tissue bring previously separated layers of coelomic mesothelium into near apposition, establishing the mesentery of the ovary, the mesovarium. Likewise, mesonephric degeneration along the region of differentiation of the unfused cranial segment of the paramesonephric ducts establishes the mesosalpinx. Caudally, growth and fusion ventromedially of these bilateral ducts “sweep” the once medially attached mesenteries of the ducts toward the midline. These bilateral mesenteries merge over the fused uterovaginal primordium and extend laterally to the pelvic wall to form a continuous double-layered “drape,” the mesometrium of the broad ligament, between the upper portion of the primordium and the posterolateral body wall. This central expanse of mesentery creates the rectouterine and vesicouterine pouches. The midline caudal fusion of the ducts also alters the previous longitudinal orientation of the upper free segments of the ducts (the oviducts) to a near transverse orientation. During this alteration, the attached mesovarium is drawn from a medial relationship into a posterior relationship with the paramesonephric mesentery of the mesosalpinx and the mesometrium.
The suspensory ligament of the ovary, through which the ovarian vessels, nerves, and lymphatics traverse, forms when cranial degeneration of the mesonephric tissue and regression of the urogenital ridge adjacent to the ovary reduce these tissues to a peritoneal fold.
The round ligament of the uterus and the proper ovarian ligament are both derivatives of the gubernaculum, which originates as a mesenchymal condensation at the caudal end of the mesonephros and extends over the initially short distance to the anterior abdominal wall (see Relocation of the Testes and Ducts). As the gonad enlarges and the mesonephric tissue degenerates, the cranial attachment of the gubernaculum appears to “shift” to the inferior aspect of the ovary. Distally, growth of the fibrous gubernaculum continues into the inguinal region. However, the midportion of the gubernaculum becomes attached, inexplicably, to the paramesonephric duct at the uterotubal junction. Formation of the uterovaginal primordium by caudal fusion of the paramesonephric ducts apparently carries the attached gubernaculum medially within the cover of the encompassing mesentery of the structures (ie, the parts of the developing broad ligament). This fibrous band of connective tissue eventually becomes 2 ligaments.
Cranially, the band is the proper ligament of the ovary, extending between the inferior pole of the ovary and the lateral wall of the uterus just inferior to the oviduct. Caudally, it continues as the uterine round ligament from a point just inferior to the proper ovarian ligament and extending through the inguinal canal to the labium majus.
D. Anomalies of the Uterine Tubes (Oviducts, Fallopian Tubes)
The uterine tubes are derivatives of the cranial segments of the paramesonephric (müllerian) ducts, which differentiate in the urogenital ridge between the sixth and ninth weeks (Fig. 2–10). Ductal formation begins with invagination of the coelomic epithelium in the lateral coelomic bay. The initial depression remains open to proliferate and differentiate into the ostium (Fig. 2–10). Variable degrees of duplication of the ostium sometimes occur; in such cases, the leading edges of the initial ductal groove presumably did not fuse completely or anomalous proliferation of epithelium around the opening occurred.
Absence of a uterine tube is very rare when otherwise normal ductal and genital derivatives are present. This anomaly has been associated with (1) ipsilateral absence of an ovary and (2) ipsilateral unicornuate uterus (and probable anomalous broad ligament). Bilateral absence of the uterine tubes is most frequently associated with lack of formation of the uterus and anomalies of the external genitalia. Interestingly, absence of the derivatives of the lower part of the müllerian ducts with persistence of the uterine tubes occurs more frequently than the reverse condition. This might be expected, as the müllerian ducts form in a craniocaudal direction.
Partial absence of a uterine tube (middle or caudal segment) also has been reported. The cause of partial absence is unknown, although several theories have been advanced. One theory holds that when the unilateral anomaly coincides with ipsilateral ovarian absence, a “vascular accident” might occur following differentiation of the ducts and ovaries. Obviously, various factors resulting in somewhat localized atresia could be proposed. From a different perspective, bilateral absence of the uterine tubes as an associated disorder in a female external phenotype is characteristic of testicular feminization syndrome or androgen insensitivity syndrome (nonpersistence of the rest of the paramesonephric ducts, anomalous external genitalia, hypoplastic male genital ducts, and testicular differentiation with usual ectopic location).
E. Anomalies of the Uterus
The epithelium of the uterus and cervix and the fibromuscular wall of the vagina are derived from the paramesonephric (müllerian) ducts, the caudal ends of which fuse medially to form the uterovaginal primordium. Most of the primordium gives rise to the uterus (Fig. 2–13). Subsequently, the caudal tip of the primordium contacts the pelvic part of the urogenital sinus, and the interaction of the sinus (sinovaginal bulbs) and primordium leads to differentiation of the vagina. Various steps in this sequential process can go awry, such as (1) complete or partial failure of one or both ducts to form (agenesis), (2) lack of or incomplete fusion of the caudal segments of the paired ducts (abnormal uterovaginal primordium), or (3) failure of development after successful formation (aplasia or hypoplasia). Many types of anomalies may occur because of the number of sites for potential error, the complex interactions necessary for the development of the müllerian derivatives, and the duration of the complete process.
Complete agenesis of the uterus is very rare, and associated vaginal anomalies are usually expected. Also, a high incidence of associated structural or positional abnormalities of the kidney has been reported; there has been speculation that the initial error in severe cases may be in the development of the urinary system and then in the formation of the paramesonephric ducts.
Aplasia of the paramesonephric ducts (müllerian aplasia) is more common than agenesis and could occur after formation and interaction of the primordium with the urogenital sinus. A rudimentary uterus or a vestigial uterus (ie, varying degrees of fibromuscular tissue present) is most frequently accompanied by partial or complete absence of the vagina. As in uterine agenesis, ectopic kidney or absence of a kidney is frequently associated with uterine aplasia (in about 40% of cases). Uterine hypoplasia variably yields a rudimentary or infantile uterus and is associated with normal or abnormal uterine tubes and ovaries. Unilateral agenesis or aplasia of the ducts gives rise to uterus unicornis, whereas unilateral hypoplasia may result in a rudimentary horn that may or may not be contiguous with the lumen of the “normal” horn (uterus bicornis unicollis with one unconnected rudimentary horn; Fig. 2–14). The status of the rudimentary horn must be considered for potential hematometra, or blood in the uterus that cannot exit, at puberty.
Figure 2–14. Uterine anomalies. (*Redrawn and reproduced, with permission, from Toaff R. A major genital malformation: communicating uteri. Obstet Gynecol 1974;43:221.)
Anomalous unification caudally of the paramesonephric ducts results in many uterine malformations (Fig. 2–14). The incidence of defective fusion is estimated to be 0.1–3% of females. Furthermore, faulty unification of the ducts has been cited as the primary error responsible for most anomalies of the female genital tract. Partial or complete retention of the apposed walls of the paired ducts can produce slight (uterus subseptus unicollis) to complete (uterus bicornis septus) septal defects in the uterus. Complete failure of unification of the paramesonephric ducts can result in a double uterus (uterus didelphys) with either a single or double vagina.
F. Anomalies of the Cervix
Because the cervix forms as an integral part of the uterus, cervical anomalies are often the same as uterine anomalies. Thus, absence or hypoplasia of the cervix is rarely found with a normal uterovaginal tract. The cervix appears as a fibrous juncture between the uterine corpus and the vagina.
Corbetta S, Muzza M, Avagliano L, et al. Gonadal structures in a fetus with complete androgen insensitivity syndrome and persistent Müllerian derivatives: comparison with normal fetal development. Fertil Steril2011;95:1119.e9–e14. PMID: 20971460.
Dighe M, Moshiri M, Phillips G, Biyyam D, Dubinsky T. Fetal genitourinary anomalies—a pictorial review with postnatal correlation. Ultrasound Q 2011;27:7–21. PMID: 21343799.
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DIFFERENTIATION OF THE UROGENITAL SINUS
Until differentiation of the genital ducts begins, the urogenital sinus appears similar in both sexes during the middle and late embryonic period. For purposes of describing the origin of sinusal derivatives, the sinus can be divided into 3 parts: (1) the vesical part, or the large dilated segment superior to the entrance of the mesonephric ducts; (2) the pelvic part, or the narrowed tubular segment between the level of the mesonephric ducts and the inferior segment; and (3) the phallic part, often referred to as the definitive urogenital sinus (the anteroposteriorly elongated, transversely flattened inferiormost segment) (Fig. 2–8). The urogenital membrane temporarily closes the inferior limit of the phallic part. The superior limit of the vesical part becomes delimited by conversion of the once tubular allantois to a thick fibrous cord, the urachus, by about 12 weeks. After differentiation of the vesical part of the sinus to form the epithelium of the urinary bladder, the urachus maintains its continuity between the apex of the bladder and the umbilical cord and is identified postnatally as the median umbilical ligament. Various anomalies of urachal formation can present as urachal fistula, cyst, or sinus, depending on the degree of patency that persists during obliteration of the allantois.
In both sexes, the caudal segments of each mesonephric duct between the urogenital sinus and the level of the ureter of the differentiating metanephric diverticulum (or ureteric bud) become incorporated into the posterocaudal wall of the vesical part (ie, urinary bladder) of the sinus (Figs. 2–5 and 2–6). As the dorsal wall of the bladder grows and “absorbs” these caudal segments, the ureters are gradually “drawn” closer to the bladder and eventually open directly and separately into it, dorsolateral to the mesonephric ducts (Figs. 2–6 and 2–7). The mesodermal segment of mesonephric duct incorporated into the bladder defines the epithelium of the trigone of the bladder, although this mesodermal epithelium is secondarily replaced by the endodermal epithelium of the sinusal bladder. After formation of the trigone, the remainder of each mesonephric duct (ie, the portion that was cranial to the metanephric diverticulum) is joined to the superior end of the pelvic part of the urogenital sinus. Thereafter, the ducts either degenerate (in females) or undergo differentiation (in males).
Male: Urinary Bladder, Urethra, & Penis (Fig. 2–15)
The urogenital sinus gives rise to the endodermal epithelium of the urinary bladder, the prostatic and membranous urethra, and most of the spongy (penile) urethra (except the glandular urethra). Outgrowths from its derivatives produce epithelial parts of the prostate and bulbourethral glands (Fig. 2–15). The prostatic urethra receives the ejaculatory ducts (derived from the mesonephric ducts) and arises from 2 parts of the urogenital sinus. The portion of this urethral segment superior to the ejaculatory ducts originates from the inferiormost area of the vesical part of the sinus. The lower portion of the prostatic urethra is derived from the pelvic part of the sinus near the entrance of the ducts and including the region of the sinusal tubercle—the latter apparently forming the seminal colliculus. Early in the 12th week, endodermal outgrowths of the prostatic urethra form the prostatic anlage, the prostatic buds, from which the glandular epithelium of the prostate will arise. Differentiation of splanchnic mesoderm contributes other components to the gland (smooth muscle and connective tissue), as is the case for mesodermal parts of the urinary bladder. The pelvic part of the sinus also gives rise to the epithelium of the membranous urethra, which later yields endodermal buds for the bulbourethral glands. The phallic, or inferior, part of the urogenital sinus proliferates anteriorly as the external genitalia form (during weeks 9–12) and results in incorporation of this phallic part as the endodermal epithelium of the spongy (penile) urethra (the distal glandular urethra is derived from ectoderm).
Figure 2–15. Schematic drawing of male differentiation of the urogenital sinus; formation of urinary bladder and urethra. (Explanatory symbols are given in Fig. 2–1.)
Early masculinization of the undifferentiated or indifferent genitalia takes place during the first 3 weeks of the fetal period (weeks 9–12) and is caused by androgenic stimulation. The phallus and urogenital folds gradually elongate to initiate development of the penis. The subjacent endodermal lining of the inferior part (phallic) of the urogenital sinus extends anteriorly along with the urogenital folds, creating an endodermal plate, the urethral plate. The plate deepens into a groove, the urethral groove, as the urogenital folds (now called urethral folds) thicken on each side of the plate. The urethral groove extends into the ventral aspect of the developing penis, and the bilateral urethral folds slowly fuse in a posterior to anterior direction over the urethral groove to form the spongy (penile) urethra, thereby closing the urogenital orifice (Fig. 2–15; see also Fig. 2–20). The line of fusion becomes the penile raphe on the ventral surface of the penis.
As closure of the urethral folds approaches the glans, the external urethral opening on this surface is eliminated. Concurrently, an ectodermal glandular plate invaginates the tip of the penis. Canalization of the plate forms the distal end of the penile urethra, the glandular urethra. Thus, the external urethral meatus becomes located at the tip of the glans when closure of the urethral folds is completed (see Fig. 2–20). The prepuce is formed slightly later by a circular invagination of ectoderm at the tip of the glans penis. This cylindric ectodermal plate then cleaves to leave a double-layered fold of skin extending over the glans.
While the cloacal folds and phallic urogenital sinus were differentiating into the penis and the urethra, the genital (labioscrotal) swellings of the undifferentiated stage were enlarging lateral to the cloacal folds. Medial growth and fusion of the scrotal swellings to form the scrotum and scrotal raphe around the 12th week virtually complete the differentiation of the male external genitalia (see Figs. 2–20 and 2–22).
Female: Urinary Bladder, Urethra, & Vagina
A. Development
Differentiation of the female sinus is schematically presented in Figure 2–13 and illustrated in Figures 2–8, 2–12, 2–16, and 2–17. In contrast to sinusal differentiation in the male, the vesical part of the female urogenital sinus forms the epithelium of the urinary bladder and entire urethra. Derivatives of the pelvic part of the sinus include the epithelium of the vagina, the greater vestibular glands, and the hymen. Controversy exists about how the vagina is formed, mainly because of a lack of consensus about the origin and degree of inclusion of its precursory tissues (mesodermal paramesonephric duct, endodermal urogenital sinus, or even mesonephric duct). The most common theory is that 2 endodermal outgrowths, the sinovaginal bulbs, of the dorsal wall of the pelvic part of the urogenital sinus form bilateral to and join with the caudal tip of the uterovaginal primordium (fused paramesonephric ducts) in the area of the sinusal tubercle (Fig. 2–12). This cellular mass at the end of the primordium occludes the inferior aspect of the canal, creating an endodermal vaginal plate within the mesodermal wall of the uterovaginal primordium. Eventually, the vaginal segment grows, approaching the vestibule of the vagina. The process of growth has been described either as “downgrowth” of the vaginal segment away from the uterine canal and along the urogenital sinus or, more commonly, as “upgrowth” of the segment away from the sinus and toward the uterovaginal canal. In either case, the vaginal segment is extended between the paramesonephric-derived cervix and the sinus-derived vestibule (Figs. 2–12, 2–16, and 2–17). Near the fifth month, the breakdown of cells centrally in the vaginal plate creates the vaginal lumen, which is delimited peripherally by the remaining cells of the plate as the epithelial lining of the vagina. The solid vaginal fornices become hollow soon after canalization of the vaginal lumen is complete. The upper one-third to four-fifths of the vaginal epithelium has been proposed to arise from the uterovaginal primordium, whereas the lower two-thirds to one-fifth has been proposed as a contribution from the sinovaginal bulbs.
Figure 2–16. Sagittal cutaway view of developing vagina and urethra.
Figure 2–17. Sagittal cutaway view of differentiated urogenital sinus and precanalization stage of vaginal development. The drawing depicts one of several theories about the relative contributions of paramesonephric ducts and sinovaginal bulbs to the vagina.
The fibromuscular wall of the vagina is derived from the uterovaginal primordium. The cavities of the vagina and urogenital sinus are temporarily separated by the thin hymen, which probably is a mixture of tissue derived from the vaginal plate and the remains of the sinusal tubercle. With concurrent differentiation of female external genitalia, inferior closure of the sinus does not occur during the 12th week of development, as it does in the male. Instead, the remainder of the pelvic part and all of the inferior phallic part of the urogenital sinus expand to form the vestibule of the vagina. Presumably, the junctional zone of pigmentation on the labia minora represents the distinction between endodermal derivation from the urogenital sinus (medially) and ectodermal skin (laterally).
B. Anomalies of the Vagina
The vagina is derived from interaction between the uterovaginal primordium and the pelvic part of the urogenital sinus (Fig. 2–13; see Development). The causes of vaginal anomalies are difficult to assess because integration of the uterovaginal primordium and the urogenital sinus in the normal differentiation of the vagina remains a controversial subject. Furthermore, an accurate breakdown of causes of certain anomalous vaginal presentations, as with many anomalies of the external genitalia, would have to include potential moderating factors of endocrine and genetic origin as well.
The incidence of absence of the vagina due to suspected vaginal agenesis is about 0.025%. Agenesis may be due to failure of the uterovaginal primordium to contact the urogenital sinus. The uterus is usually absent (Fig. 2–18). Ovarian agenesis is not usually associated with vaginal agenesis. The presence of greater vestibular glands has been reported with presumed vaginal agenesis; their presence emphasizes the complexity of differentiation of the urogenital sinus.
Figure 2–18. Midsagittal view of vaginal agenesis and uterine agenesis with normal ovaries and oviducts.
Vaginal atresia, on the other hand, is considered when the lower portion of the vagina consists merely of fibrous tissue while the contiguous superior structures (the uterus, in particular) are well differentiated (perhaps because the primary defect is in the sinusal contribution to the vagina). In müllerian aplasia almost all of the vagina and most of the uterus are absent (Rokitansky-Küster-Hauser syndrome, with a rudimentary uterus of bilateral, solid muscular tissue, was considered virtually the same as this aplasia). Most women with absence of the vagina (and normal external genitalia) are considered to have müllerian aplasia rather than vaginal atresia.
Other somatic anomalies are sometimes associated with müllerian aplasia, suggesting multiple malformation syndrome. Associated vertebral anomalies are much more prevalent than middle ear anomalies, eg, müllerian aplasia associated with Klippel-Feil syndrome (fused cervical vertebrae) is more common than müllerian aplasia associated with Klippel-Feil syndrome plus middle ear anomalies (“conductive deafness”). Winter’s syndrome, which is thought to be autosomal recessive, is evidenced by middle ear anomalies (somewhat similar to those in the triad above), renal agenesis or hypoplasia, and vaginal atresia (rather than aplasia of the paramesonephric ducts). Dysgenesis(partial absence) of the vagina and hypoplasia (reduced caliber of the lumen) have also been described.
Transverse vaginal septa (Fig. 2–19) are probably not the result of vaginal atresia but rather of incomplete canalization of the vaginal plate or discrete fusion of sinusal and primordial (ductal) derivatives. Alternative explanations are likely because the histologic composition of septa is not consistent. A rare genetic linkage has been demonstrated. A single septum or multiple septa can be present, and the location may vary in upper or lower segments of the lumen. Longitudinal vaginal septa can also occur. A variety of explanations have been advanced, including true duplication of vaginal primordial tissue, anomalous differentiation of the uterovaginal primordium, abnormal variation of the caudal fusion of the müllerian ducts, persistence of vaginal plate epithelium, and anomalous mesodermal proliferation. Septa may be imperforate or perforated. A transverse septum creates the potential for various occlusive manifestations (eg, hydrometrocolpos, hematometra, or hematocolpos), depending on the composition and location of the trapped fluid.
Figure 2–19. Transverse vaginal septa.
Abnormalities of the vagina are often associated with anomalies of the urinary system and the rectum because differentiation of the urogenital sinus is involved in formation of the bladder and urethra as well as the vagina and vestibule. Furthermore, if partitioning of the cloaca into the sinus and anorectal canal is faulty, then associated rectal defects can occur. Compound anomalies may affect the urinary tract or rectum. The urethra may open into the vaginal wall; even a single vesicovaginal cavity has been described. On the other hand, the vagina can open into a persistent urogenital sinus, as in certain forms of female pseudohermaphroditism. Associated rectal abnormalities include vaginorectal fistula, vulvovaginal anus, rectosigmoidal fistula, and vaginosigmoidal cloaca in the absence of the rectum (see Cloacal Dysgenesis).
C. Anomalies of the Hymen
The hymen is probably a mixture of tissue derived from remains of the sinusal tubercle and the vaginal plate. Usually, the hymen is patent, or perforate, by puberty, although an imperforate hymen is not rare. The imperforate condition can be the result of a congenital error of lack of central degeneration or the result of inflammatory occlusion after perforation. Obstruction of menstrual flow at puberty may be the first sign (Fig. 2–19).
D. Cloacal Dysgenesis (Including Persistence of the Urogenital Sinus)
Anomalous partitioning of the cloaca by abnormal development of the urorectal septum is rare, at least based on reported cases in the literature. As anticipated from a developmental standpoint, the incidence of associated genitourinary anomalies is high. Five types of cloacal or anorectal malformations are summarized in Table 2–3.
Table 2–3. Cloacal malformations.
Rectocloacal fistula with a persistent cloaca provides a common canal or outlet for the urinary, genital, and intestinal tracts. The distinction between a canal and an outlet is one of depth (deep versus very shallow, respectively) of the persistent lower portion of the cloaca and, thus, the length of the individual urethral and vaginal canals emptying into the cloaca. The inverse relationship between depth (or length) of the cloaca and length of the vaginal and urethral canals is probably a reflection of the time when arrest of formation of the urorectal septum occurs. Although the bladder, the vagina, and the rectum can empty into a common cloaca as just described, other unusual variations of persistent cloaca can also occur.
For example, the vagina and rectum develop, but the urinary bladder does not develop as a separate entity from the cloaca. Instead, the vagina and rectum open separately into a “urinary bladder,” which has ureters entering posterolaterally to the vagina (vaginal orifice is in the “anatomic trigone” of the bladderlike structure). The external orifice from the base of this cloacal “bladder” is a single narrow canal. One explanation for this variant might be that arrest of formation of the urorectal septum occurs much earlier than does the separate development of distal portions of the 3 tracts (urethra, vagina, and anorectum) to a more advanced (but still incomplete) stage before urorectal septal formation ceases. The anomaly is probably rare.
With a rectovaginal fistula, the vestibule may appear anatomically normal, but the anus does not appear in the perineum. The defect probably results from anorectal agenesis due to incomplete subdivision of the cloaca (similar agenesis in the male could result in a rectourethral fistula). The development of the anterior aspect of the vagina completes the separation of the urethra from the vagina, so there is not a persistent urogenital sinus. Anorectal agenesisis reputedly the most common type of anorectal malformation, and usually a fistula occurs. Rectovaginal, anovestibular (or rectovestibular; Table 2–3), and anoperineal fistulas account for most anorectal malformations.
In the absence of the anorectal defect (normal anal presentation) but presence of a persistent urogenital sinus with a single external orifice, various irregularities of the urethra and genitalia can appear. The relative positions of urethral and vaginal orifices in the sinus can even change as the child grows. In the discussion of anomalies of the labia majora, there may be a persistent urogenital sinus in female pseudohermaphroditism due to congenital adrenal hyperplasia. The vagina opens into the persisting pelvic part of the sinus, which extends with the phallic part of the sinus to the external surface at the urogenital opening. The sinus can be deep and narrow in the neonate, approximating the size of a urethra, or it can be relatively shallow.
Urinary tract disorders associated with persistent urogenital sinus include duplication of the ureters, unilateral ureteral and renal agenesis or atresia, and lack of or abnormal ascent of the kidneys. Variations in the anomalies of derivatives of the urogenital sinus appear to be related in part to the time of arrest of normal differentiation and development of the urogenital sinus, as well as to the impact of other factors associated with abnormal sexual differentiation, such as the variable degrees of response to adrenal androgen in congenital adrenal hyperplasia.
THE EXTERNAL GENITALIA
Undifferentiated Stage
The external genitalia begin to form early in the embryonic period, shortly after development of the cloaca. The progenitory tissues of the genitalia are common to both sexes, and the early stage of development is virtually the same in females and males. Although differentiation of the genitalia can begin around the onset of the fetal period if testicular differentiation is initiated, definitive genital sex is usually not clearly apparent until the 12th week. Formation of external genitalia in the male involves the influence of androgen on the interaction of subepidermal mesoderm with the inferior parts of the endodermal urogenital sinus. In the female, this androgenic influence is absent.
The external genitalia form within the initially compact area bounded by the umbilical cord (anteriorly), the developing limb buds (laterally), the embryonic tail (posteriorly), and the cloacal membrane (centrally). Two of the primordia for the genitalia first appear bilaterally adjacent to the cloacal membrane (a medial pair of cloacal folds and a lateral pair of genital [labioscrotal] swellings). The cloacal foldsare longitudinal proliferations of caudal mesenchyme located between the ectodermal epidermis and the underlying endoderm of the phallic part of the urogenital sinus. Proliferation and bilateral anterior fusion of these folds create the genital tubercle, which protrudes near the anterior edge of the cloacal membrane by the sixth week (Figs. 2–20 to 2–22). Extension of the tubercle forms the phallus, which at this stage is the same size in both sexes.
Figure 2–20. Schematic drawing of formation of male external genitalia. (Explanatory symbols are given in Fig. 2–1.)
Figure 2–21. Schematic drawing of formation of female external genitalia. (Explanatory symbols are given in Fig. 2–1.)
Figure 2–22. Development of external genitalia. A: Before sexual differentiation and just after the urorectal septum divides the cloacal membrane. B and D: Male differentiation at about 10 weeks and near term, respectively. The urogenital folds fuse ventrally over the urethral groove to form the spongy urethra and close the inferior phallic part of the urogenital sinus. The glandular urethra forms by canalization of invaginated ectoderm from the tip of the glans. C and E: Female differentiation at about 10 weeks and near term, respectively. Until about 12 weeks, there is little difference in the appearance of female and male external genitalia. The urogenital folds fuse only at their anterior and posterior extremes, while the unfused remainder differentiates into the labia minor. (See also Figs. 2–20 and 2–21.)
By the seventh week, the urorectal septum subdivides the bilayered (ectoderm and endoderm) cloacal membrane into the urogenital membrane (anteriorly) and the anal membrane (posteriorly). The area of fusion of the urorectal septum and the cloacal membrane becomes the primitive perineum, or perineal body. With formation of the perineum, the cloacal folds are divided transversely as urogenital foldsadjacent to the urogenital membrane and anal folds around the anal membrane. As the mesoderm within the urogenital folds thickens and elongates between the perineum and the phallus, the urogenital membrane sinks deeper into the fissure between the folds. Within a week, this membrane ruptures, forming the urogenital orifice and, thus, opening the urogenital sinus to the exterior. Similar thickening of the anal folds creates a deep anal pit, in which the anal membrane breaks down to establish the anal orifice of the anal canal (Figs. 2–20 and 2–21).
Subsequent masculinization or feminization of the external genitalia is a consequence of the respective presence or absence of androgen and the androgenic sensitivity or insensitivity of the tissues. The significance of both of these factors (availability of hormone and sensitivity of target tissue) is exemplified by the rare condition (about 1 in 50,000 “females”) of testicular feminization, wherein testes are present (usually ectopic) and produce testosterone and antimüllerian hormone. The antimüllerian hormone suppresses formation of the uterus and uterine tubes (from the paramesonephric ducts), whereas testosterone supports male differentiation of the mesonephric ducts to form the epididymis and ductus deferens. The anomalous feminization of the external genitalia is considered to be due to androgenic insensitivity of the precursor tissues consequent to an abnormal androgen receptor or postreceptor mechanism set by genetic inheritance.
Female
A. Development of External Genitalia
Feminization of the external genitalia proceeds in the absence of androgenic stimulation (or nonresponsiveness of the tissue). The 2 primary distinctions in the general process of feminization versus masculinization are (1) the lack of continued growth of the phallus and (2) the near absence of fusion of the urogenital folds and the labioscrotal swellings. Female derivatives of the indifferent sexual primordia for the external genitalia are virtually homologous counterparts of the male derivatives. Formation of the female genitalia is schematically presented in Figure 2–21.
The growth of the phallus slows relative to that of the urogenital folds and labioscrotal swellings and becomes the diminutive clitoris. The anterior extreme of the urogenital folds fuses superior and inferior to the clitoris, forming the prepuce and frenulum of the clitoris, respectively. The mid-portions of these folds do not fuse but give rise to the labia minora. Lack of closure of the folds leaves the urogenital orifice patent and results in formation of the vestibule of the vagina from the inferior portion of the pelvic part and the phallic part of the urogenital sinus at about the fifth month (Fig. 2–21). Derivatives of the vesical part of the sinus (the urethra) and the superior portion of the pelvic part of the sinus (vagina and greater vestibular glands) then open separately into the vestibule. The frenulum of the labia minora is formed by fusion of the posterior ends of the urogenital folds. The mesoderm of the labioscrotal swellings proliferates beneath the ectoderm and remains virtually unfused to create the labia majoralateral to the labia minora. The swellings blend together anteriorly to form the anterior labial commissure and the tissue of the mons pubis, while the swellings posteriorly less clearly define a posterior labial commissure. The distal fibers of the round ligament of the uterus project into the tissue of the labia majora.
B. Anomalies of the Labia Minora
In otherwise normal females, 2 somewhat common anomalies occur—labial fusion and labial hypertrophy. True labial fusion as an early developmental defect in the normally unfused midportions of the urogenital folds is purportedly less frequent than “fusion” due to inflammatory-type reactions. Labial hypertrophy can be unilateral or bilateral and may require surgical correction in extreme cases.
C. Anomalies of the Labia Majora
The labia majora are derived from the bilateral genital (labioscrotal) swellings, which appear early in the embryonic period and remain unfused centrally during subsequent sex differentiation in the fetal period. Anomalous conditions include hypoplastic and hypertrophic labia as well as different gradations of fusion of the labia majora. Abnormal fusion (masculinization) of labioscrotal swellings in genetic females is most commonly associated with ambiguous genitalia of female pseudohermaphroditism consequent to congenital adrenal hyperplasia (adrenogenital syndrome). Over 90% of females with congenital adrenal hyperplasia have a steroid 21-hydroxylase deficiency (autosomal recessive), resulting in excess adrenal androgen production. This enzyme deficiency has been reported to be “the most common cause of ambiguous genitalia in genetic females.” Associated anomalies include clitoral hypertrophy and persistent urogenital sinus. Formation of a penile urethra is extremely rare.
D. Anomalies of the Clitoris
Clitoral agenesis is extremely rare and is due to lack of formation of the genital tubercle during the sixth week. Absence of the clitoris could also result from atresia of the genital tubercle. The tubercle forms by fusion of the anterior segments of the cloacal folds. Very rarely, these anterior segments fail to fuse, and a bifid clitoris forms. This anomaly also occurs when unification of the anterior parts of the folds is restricted by exstrophy of the cloaca or bladder. Duplication of the genital tubercle with consequent formation of a double clitoris is equally rare. Clitoral hypertrophy alone is not common but may be associated with various intersex disorders.
E. Anomalies of the Perineum
The primitive perineum originates at the area of contact of the mesodermal urorectal septum and the endodermal dorsal surface of the cloacal membrane (at 7 weeks). During normal differentiation of the external genitalia in the fetal period, the primitive perineum maintains the separation of the urogenital folds and ruptured urogenital membrane from the anal folds and ruptured anal membrane, and later develops the perineal body. Malformations of the perineum are rare and usually associated with malformations of cloacal or anorectal development consequent to abnormal development of the urorectal septum. Imperforate anus has an incidence of about 0.02%. The simplest form (rare) is a thin membrane over the anal canal (the anal membrane failed to rupture at the end of the embryonic period). Anal stenosis can arise by posterior deviation of the urorectal septum as the septum approaches the cloacal membrane, causing the anal membrane to be smaller (with a relatively increased anogenital distance through the perineum). Anal agenesis with a fistula detected as an ectopic anus is considered to be a urorectal septal defect. The incidence of agenesis with a fistula is only slightly less than that without a fistula. In females, the fistula commonly may be located in the perineum (perineal fistula) or may open into the posterior aspect of the vestibule of the vagina (anovestibular fistula; see Cloacal Dysgenesis).
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