Embryology
Formation of the Mesoderm
The period of organogenesis, also called the embryonic period , spans from the third to eighth week of development. At approximately 15 days after fertilization, invagination and lateral migration of mesodermal cells occur between the ectodermal and endodermal layers of the embryo. At approximately the sixteenth day of development, the posterior wall of the yolk sac forms a small diverticulum, the allantois, which extends into the connecting stalk. The hindgut undergoes slight dilation to form the cloaca; it ventrally receives the allantois and it laterally receives the two mesonephric ducts. By the seventeenth day of development, the endoderm and ectoderm layers are separated entirely by the mesoderm layer, with the exception of the prochordal plate cephalically and the cloacal plate caudally. The cloacal plate consists of tightly adherent endodermal and ectodermal layers. With ventral bending of the embryo, the connecting stalk and contained allantois, as well as the cloacal membrane, are displaced onto the ventral aspect of the embryo.
A spur of mesodermal tissue migrates from the base of the allantois toward the cloacal membrane at approximately 28 days after fertilization, forming the urorectal septum ( Fig. 3.1 ). This structure partitions the cloaca into a ventral urogenital sinus and a dorsal rectum. After this, each of the three germ layers—the endoderm, mesoderm, and ectoderm—give rise to specific tissues and organs. The intermediate mesoderm differentiates into the urogenital structures. Other portions of the mesoderm form multiple structures including the vascular system, spleen, connective tissue, and the wall of the gut tube.
At birth, the urogenital system can be divided functionally into two different systems: the urinary system and the genital system. However, embryologically and anatomically, they are intimately connected because both develop from a common mesodermal ridge known as the intermediate mesoderm. Both systems share a common origin with significant interaction between the components of each system during the later stages of development. Epithelial-mesenchymal interactions are primarily responsible for the maintenance and differentiation of many of the structures of the urogenital system. Furthermore, male and female sexual differentiation is dependent on the presence of specific promoting or inhibiting factors.
This chapter will explore the embryology and basic congenital anomalies of the urinary tract, rectum, and anal sphincters. It will delineate the embryology of female genital organs and discuss congenital anomalies and their treatments in detail.
Urinary System
Kidney and Ureter
The urinary system begins to take shape before any gonadal development is evident. The kidney exists in three distinct but slightly overlapping forms during development. The first is the pronephros, which is present at approximately 22 days of gestation and is rudimentary and nonfunctioning. The second form, the mesonephros, likely functions for a short time during the early fetal period. The excretory tubules of the mesonephros enter the longitudinal collecting duct (primary nephric duct). These formations go on to form the mesonephric duct, which is crucial in the formation of the male genital system, but disappears in the female. By the end of the fourth week, the mesonephric ducts attach to the cloaca and a continuous lumen is present. The ureteric bud is an epithelial outgrowth of the portion of the mesonephric duct near the attachment to the cloaca. The bud penetrates the mesodermal tissue and ultimately forms the renal calyces, ureter, renal pelvis, and collecting tubules. Interactions between the mesoderm and ureteric bud form the third and permanent form of kidney, the metanephros ( Fig. 3.2 ). The excretory units of the kidney develop from the metanephric mesoderm.
The definitive kidney, formed from the metanephros, begins to function around the twelfth week of development. Urine is passed and mixes with amniotic fluid. The fetus swallows this and recycles it through the kidney. During fetal life, it is the placenta that is responsible for waste excretion, not the kidney.
The kidney is initially positioned in the pelvic region, but it begins a caudal ascent secondary to lumbar and sacral body growth in the late embryonic and early fetal periods. If a kidney fails to ascend, then it remains close to the iliac vessels and is known as a pelvic kidney. The kidneys are occasionally so close in proximity that the lower poles fuse, leading to development of a horseshoe kidney. This structure often cannot ascend completely because it cannot pass the inferior mesenteric artery root.
Bladder, Trigone, and Urethra
During the fourth to seventh week of development, the cloaca divides into two separate structures: the urogenital sinus (anteriorly) and the anal canal (posteriorly). These structures are separated by a mesodermal structure, the urorectal septum. The tip of this septum eventually forms the perineal body. The upper and largest portion of the urogenital sinus forms the urinary bladder, which is of endodermal origin. The bladder is initially continuous with the allantois (rudimentary structure primarily involved in nutrition and waste excretion in the embryo), but the lumen of the allantois becomes obliterated and forms a fibrous cord called the urachus, which runs from the bladder dome to the umbilicus and is called the median umbilical ligament in the adult. The middle portion of the urogenital sinus forms the prostatic and membranous parts of the urethra, and the distal portion forms the phallic portion.
The expanding wall of the bladder grows and incorporates the mesonephric ducts and ureteric buds, forming the bladder trigone. This structure provides a mesodermal contribution to the endodermal wall of the vesicourethral canal. At approximately 42 days after fertilization, the trigone may be defined as the region of the vesicourethral canal lying between the ureteric orifices and the termination of the mesonephric ducts ( Fig. 3.2 ). The caudal portion of the vesicourethral canal remains narrow and forms the entire urethra. The urethra consists of an epithelium that is derived from endoderm whereas the surrounding connective tissue and smooth muscle is mesodermal in origin. In females, the cranial portion of the urethra gives rise to the urethral and paraurethral glands. A timetable and schematic representation of the embryologic contributions of the various structures of the urogenital system are shown in Table 3.1 and Figure 3.3 , respectively.
| 15 days | Ingrowth of intraembryonic mesoderm |
| 16-17 days | Allantois appears |
| 17 days | Cloacal plate forms |
| 22 days | Pronephros present |
| 28 days | Mesonephric duct reaches cloaca; ureteric bud appears |
| 28-49 days | Partitioning of cloaca by urorectal septum |
| 30-37 days | Ureteric bud initiates formation of metanephros (permanent kidney) |
| 41 days | Lumen of urethra is discrete; genital tubercles prominent |
| 42-44 days | Mesonephric ducts and ureters drain separately into urogenital sinus, defining boundaries of trigone |
| 51-52 days | Kidneys in lumbar region; glomeruli appear in kidney |
| 12 weeks | Renal function initiates |
| 13 weeks | Bladder becomes muscularized |
| 20-40 weeks | Further growth and development complete the urogenital organs |
The separate development of the trigone and bladder may explain why the muscle laminae of the trigone are contiguous with the muscle of the ureter, but not with the detrusor muscle of the bladder. This separate development also may account for pharmacologic responses of the musculature of the bladder neck and trigone, which partially differ from those of the detrusor. Because of its embryonic origins (mesonephric ducts and ureters), the bladder mucosa is initially mesodermal, but with time, this lining is replaced by endodermal epithelia from the hindgut.
Congenital Anomalies of the Urinary Tract
Anomalies of the urinary system are common (3% to 4% of live births). Knowledge of the embryology of the genitourinary system is necessary for understanding the causes of the multiple congenital anomalies of the upper and lower urinary tracts. Selected congenital anomalies of the urinary tract and their embryologic causes are shown in Table 3.2 .
| Condition | Embryologic Cause |
|---|---|
| Renal agenesis | Faulty interaction between the ureteric bud and metanephrogenic mesenchyme |
| Pelvic kidney | Failure of kidney to ascend to the lumbar region |
| Horseshoe kidney | Fusion of lower poles of both kidneys; ascent to lumbar region prevented by root of inferior mesenteric artery |
| Urachal fistula, cyst, sinus | Variable persistence of the intraembryonic portion of allantois; from bladder to umbilicus |
| Double ureter | Early splitting of the ureteric bud |
| Ectopic ureter | Two ureteric buds develop from one mesonephric duct. One bud is in normal position; the abnormal bud moves downward with the mesonephric duct to enter into the urethra, vagina, vestibule, or uterus |
| Bladder exstrophy | Insufficiency of mesoderm of the ventral abdominal wall leading to abdominal ectodermal breakdown and degeneration of the anterior bladder wall; the bladder opens broadly onto the abdominal wall |
Rectum and Anal Sphincters
Normal Development of the Hindgut
In the early embryo, the caudal portion of the primitive gut forms the hindgut. The hindgut extends from the posterior intestinal portal to the cloacal membrane and gives rise to the distal third of the transverse colon, the descending colon, the sigmoid, the rectum, and the upper part of the anal canal. The terminal portion of the hindgut enters into the cloaca, an endoderm-lined cavity that is in direct contact with the surface ectoderm. This boundary between the endoderm and ectoderm forms the cloacal membrane.
During further development, a transverse ridge called the urorectal septum arises from mesoderm between the allantois and the hindgut. This septum gradually grows caudally, thereby dividing the cloaca into an anterior portion, the primitive urogenital sinus, and a posterior portion, the anorectal canal. The primitive perineum is formed when the urorectal septum reaches the cloacal membrane when the embryo is 7 weeks. At this time, the cloacal membrane ruptures, creating an anal opening for the hindgut and a ventral opening for the urogenital sinus.
Ectoderm on the surface portion of the cloaca proliferates and invaginates to create the anal pit. Degeneration of the anal membrane (formerly cloacal membrane) establishes continuity between the upper and lower anal canal. Thus, the upper part of the anal canal is endodermal in origin; the lower third of the anal canal is ectodermal. The junction between these regions is delineated by the pectinate or dentate line. The embryologic origin of the anus explains the differing blood supplies and epithelial cell types of the upper and lower anal canal.
The external anal sphincter appears in human embryos at approximately 7 to 8 weeks. This sphincter, together with the levator ani, is believed to originate from hypaxial myotomes. Although the anal sphincter and levator ani may arise from distinct primordia, their relationship is very close.
Congenital Anomalies of the Rectum and Anal Sphincters
Imperforate anus is one of the more common abnormalities of the hindgut. In simple cases, the anal canal ends blindly at the anal membrane, which then forms a diaphragm between the endodermal and ectodermal portions of the anal canal. This occurs when the anal membrane fails to break down. In more severe cases, a thick layer of connective tissue may be found between the terminal end of the rectum and the surface because of either a failure of the anal pit to develop or atresia of the ampullar part of the rectum. Vascular accidents in this region are the likely cause of rectoanal atresias. These vary greatly and present as a fibrous remnant or loss of a segment of the rectum or anus.
Rectourethral and rectovaginal fistulas are likely caused by abnormalities in the formation of the cloaca or urorectal septum. If the urorectal septum does not extend far enough caudally or it shifts anteriorly, then the hindgut opens into the urethra or vagina, creating a fistula.
Female Genital System
Gonads
Gonads appear in early development as a pair of longitudinal ridges of steroidgenic mesoderm called the genital or gonadal ridges , which are formed by proliferation of the epithelium and condensation of the underlying mesenchyme. These initially do not contain germ cells. Primordial germ cells originate in the epiblast, migrate through the primitive streak, and by the third week reside in the posterior wall of the yolk sac ( Fig. 3.4 ). They then migrate along the dorsal mesentery of the hindgut and arrive at the primitive gonads at the beginning of the fifth week, and they invade the genital ridges in the sixth week. Approximately 1000 to 2000 primordial germ cells enter the genital ridges. If the primordial germ cells fail to reach the genital ridge, then the gonads do not develop. In the female, once the primordial germ cells reach the genital ridge, they are called oogonia . Oogonia begin to proliferate at this stage which continues into the fourth month. Once these cells proliferate via mitosis, they are called oocytes .
In the absence of the Y chromosome (XX or XO) and in the presence of the appropriate somatic cell environment, the primitive sex cords (medullary cords) regress ( Fig. 3.5 A ). However, the surface epithelium continues to proliferate and develops a second generation of cords called cortical cords . Cells in these cords surround each oocyte with a layer of epithelial cells called follicular cells . Together, the oocyte and follicular cells constitute a primary follicle ( Fig. 3.5 B ). These enter meiosis until prophase I, when meiosis is arrested. The oocytes remain in this phase until ovulation, which may be up to approximately 50 years later.
Genital Ducts, Sexually Indifferent Stage
As in gonadal development, the embryo genital ducts go through a sexually indifferent stage. Until approximately 8 weeks, male and female embryos contain two pairs of genital ducts: paramesonephric (müllerian) ducts and mesonephric (wolffian) ducts. The paramesonephric ducts are a longitudinal invagination of the epithelium of the urogenital ridge. The ducts open cranially into the intra-abdominal cavity and extend caudally. The paramesonephric ducts travel lateral to the mesonephric ducts and move medially, crossing the mesonephric ducts ventrally and coming into close proximity in the midline. The paired paramesonephric ducts initially are separated by a septum but later fuse to form the uterine canal. Once these ducts are combined, the fused tip projects into the posterior wall of the urogenital sinus where it causes a small swelling called the paramesonephric tubule. The mesonephric ducts open into the urogenital sinus on either side of this tubercle.
Sexual Differentiation
Sexual differentiation is determined genetically at the time of fertilization, but the gonads do not acquire sex-specific morphologic characteristics until the seventh week of development. Sexual differentiation is a complex process that depends on the presence or absence of the Y chromosome, which contains the testis-determining gene, SRY ( sex determining region on Y ). Under its influence, male development occurs; in its absence, female development occurs.
The way in which the genital ducts undergo sexual differentiation is an intricate process that involves multiple levels of molecular regulation, of which only part is understood. The SRY protein, autosomal gene SOX9 , and müllerian inhibiting substance all play important roles. SRY is a transcription factor that acts in conjunction with the autosomal gene SOX9 . When the Y chromosome is present, SRY and SOX9 influence multiple pathways that lead to testes development and testosterone production in the male. SOX9 is also known to bind to the promoter region of the gene for müllerian inhibiting substance (MIS). MIS is critical in sexual differentiation because its presence leads to regression of the paramesonephric ducts ( Fig. 3.6 ). When the Y chromosome is not present, WNT4 , called the ovary-determining gene , inhibits the function of SOX9 and regulates the expression of other genes responsible for ovarian differentiation. Estrogens are also important in sexual differentiation because they stimulate the paramesonephric ducts to form the uterus, fallopian tubes, cervix, and upper vagina. They also act on external genitalia.
Once sexual differentiation begins at approximately 7 to 8 weeks, the paramesonephric ducts develop into three main sections: (1) the cranial portion that opens into the abdominal cavity, (2) the horizontal portion that crosses the mesonephric duct, and (3) the fused caudal portion located medial to the mesonephric duct ( Fig. 3.7 A ). As the paramesonephric ducts move medially, a pelvic fold is formed from the pelvis to the fused paramesonephric ducts, forming the broad ligament. The two most cranial portions develop into the fallopian tubes, and the fused distal portion forms the uterus and cervix ( Fig. 3.7 B ).
Vagina
After the solid tip of the paramesonephric ducts reaches the urogenital sinus, two solid evaginations, known as the sinovaginal bulbs, grow out from the pelvic part of the sinus ( Fig. 3.8 A ). These bulbs proliferate and form the solid vaginal plate. Proliferation continues at the cranial plate, which increases the distance between the uterus and urogenital sinus ( Fig. 3.8 B ). The solid plate becomes canalized by the fifth month of development ( Fig. 3.8 C ). This demonstrates that the vagina has two origins: the upper vagina originates from the paramesonephric ducts and the lower vagina from the urogenital sinus. The lumen of the vagina remains separated from the urogenital sinus by the hymen, which consists of the epithelial lining of the sinus and a thin layer of vaginal cells.
Although the mesonephric ducts regress in a developing female embryo, in some females, a remnant of these ducts persists. If a small cranial portion persists, then it forms the epoohoron or the paroophoron. Most of these regress during further embryonic development but the most cranial portion (found in the epoophoron) and most caudal portion (found in the wall of uterus or vagina and called Gartner’s cyst ) may be found in adult women ( Fig. 3.7 B ).
External Genitalia
Cloacal folds form around the cloacal membrane in the third week of development. Cranial to the cloacal membrane, these folds fuse to form the genital tubercle ( Fig. 3.9 A ). Caudally, the folds are subdivided into urogenital folds anteriorly and anal folds posteriorly. A second set of tissue elevations, the genital swellings, becomes visible on each side of the urogenital folds ( Fig. 3.9 B ). When the original cloacal membrane breaks down during the eighth week, the urogenital sinus opens between the genital folds directly to the outside. Estrogens further stimulate development of the external genitalia. The genital tubercle elongates and forms the clitoris. The urogenital folds form the labia minora, and the genital swellings form the labia majora. The urogenital sinus remains open and forms the vestibule, into which the urethra and vagina open ( Fig. 3.9 C ). Table 3.3 delineates the embryologic origin of the major female genital structures.
| Indifferent Structure | Female Derivative |
|---|---|
| Genital ridge | Ovary |
| Primordial germ cells | Ova |
| Sex cords | Follicular (granulosa) cells |
| Mesonephric tubules | Oophoron |
| Paroophoron | |
| Mesonephric ducts | Appendix of ovary |
| Gartner’s duct cyst | |
| Paramesonephric ducts | Fallopian tubes |
| Uterus | |
| Upper vagina | |
| Definitive urogenital sinus (lower part) | Lower vagina |
| Vulvar vestibule | |
| Early urogenital sinus (upper part) | Urinary bladder |
| Urethra | |
| Genital tubercle | Clitoris |
| Urogenital folds | Labia minora |
| Genital swellings | Labial majora |
Congenital Anomalies of the Female Genital System
Congenital Anomalies of the Female Genital System Presenting as Congenital Absence of the Vagina and Uterus
Malformations of the müllerian ducts and the external genitalia can have a significant effect on reproductive potential and sexual function. When a patient presents with such an abnormality, it is important to put significant thought and time into determining the correct diagnosis and subsequent treatment.
The literature reports the rate of female congenital anomalies to be between 0.2% and 0.4% of the general population. However, the prevalence of female congenital anomalies may be as high as 7% when using some of the newer diagnostic methods. These rates are much higher when looking at subgroups of patients with recurrent pregnancy loss and infertility. However, in many instances these anomalies are asymptomatic.
Although sex steroid receptor abnormalities and enzyme deficiencies are not considered malformations of the müllerian ducts, they often have similar clinical presentations and should be considered in the differential. The workup and diagnosis of these disorders are beyond the scope of this chapter; however, similar reconstructive techniques are often utilized.
Classification
The classification of müllerian anomalies helps with the diagnosis and the comparison of outcomes after various modes of management. However, there is no single classification that encompasses all anomalies that have been reported in the literature.
Although the direct cause of most of these anomalies is not known, on the basis of our embryologic knowledge, the pathogenesis of most of these anomalies can be understood. On the basis of pathophysiology, müllerian anomalies can be broadly classified as being related to (1) agenesis, (2) vertical fusion defects, or (3) lateral fusion defects. Compared with other anomalies, agenesis of the uterus and vagina are relatively common abnormalities. Vertical fusion defects are usually the result of abnormal canalization of the vaginal plate and result in defects such as a transverse vaginal septum and imperforate hymen. Lateral fusion defects can be symmetrical or asymmetrical and include septum of the uterus and vagina as well as unicornuate and bicornuate uteri and related abnormalities.
There are endless variations to müllerian and vaginal anomalies. It is impossible to effectively note these variations in any one classification system. The most accepted classification of uterine anomalies, published by the American Society of Reproductive Medicine (ASRM), places uterine anomalies into distinct groups on the basis of anatomic configuration ( Table 3.4 ). Because vaginal anomalies are not included in this classification, they must be described along with the uterine anomaly. This classification does not give insight into pathophysiology, but it is an effective way to communicate observations for the purposes of treatment and prognosis. Other, more comprehensive classification systems do exist, however, they are utilized less in the United States.
