Development of the Urogenital Tract
Development of the Genital Tract
The “default” setting for development of the reproductive system is female.
The presence of the Y-chromosome in male individuals is the key to sexual dimorphism because the Y-chromosome possesses the testis determining gene SRY. The protein product of SRY is testis determining factor(TDF) which is a transcription factor that initiates the process by causing the cells of the genital ridge to form testicular structures along with the production of testosterone.
The reproductive system begins development in the intermediate mesoderm of the posterior body wall. The genital (gonadal) ridges form from thickenings of mesoderm on the posterior body wall.
In addition to the genital ridge, two sets of parallel ducts develop in all embryos. These differentiate into the major ducts of the male and female reproductive tracts. Their development is guided by factors produced by the developing gonads.
On the surface of the embryo, everal external primordia develop to form the external genitalia.
Development of the Gonads
Primordial germ cells originate in the epiblast layer and in week 3 of development, migrate through the primitive streak to be interspersed among the endodermal cells of the wall of the yolk sac (purple circle).
During the fourth week of development, these cells migrate from the yolk sac through the dorsal mesentery to seed the primitive sex cords in the mesoderm of the gonadal (genital) ridges.
In a 5-week embryo, the gonadal ridge is adjacent to the mesonephric kidney. A cross section taken through the region indicated by the red line shows migration of primordial germ cells from the posterior wall of the yolk sac and through the dorsal mesentery of the hidngut to reach and seed the primitive sext cords of the gonadal ridge.
In the presence of a Y chromosome and TDF, the gonadal ridge forms a testis. In the absence of the Y chromosome, estrogen and progesterone from maternal and placental sources guide the development of the gonadal ridge to become an ovary.
Ovary Development
In female embryos, which possess a pair of X sex chromosomes, the primitive sex cords break into clusters containing primitive germ cells and which are located within the medulla of the genital ridge. These degenerate and are replaced by vascular tissue forming the medullary stroma of the ovary. A second wave of sex cords develops adjacent to the surface of the developing gonad during the 7th week. These are the cortical sex cords and they prevail to come the primordial follicles of the ovary.
In both males and females, the gonads descend from their initial position of origin on the posterior body wall to a final distal location. Each developing gonad is attached at its inferior pole to a fibromuscular band of tissue called the gubernaculum. The distal end of the gubernaculum is anchored in subcutaneous tissue of the labioscortal swelling. The gubernaculum passes through the inguinal canal to reach the labioscrotal swelling.
As the gubernaculum shortens, the ovary is guided toward the inguinal canal. However, about the time it reaches the level of the brim of the pelvis, the expansion of the developing uterus from the paramesonephric ducts traps the gubernaculum in the wall of the uterus, arresting the descent of the ovary at the level of the pelvis. The part of the gubernaculum between the ovary and the wall of the uterus persists in the adult as the proper ovarian ligament. The part of the gubernaculum that extends from the uterus though the inguinal canal to the labium majus is the round ligament of the uterus.
Testis Development
In the gonadal ridge of genetic males, the presence of a Y chromosome in the primordial germ cells will guide the development of the indifferent gonad to become a testis. Testis determining factor is encoded by the SRY gene of the Y chromosome. This results in proliferation of the medullary sex cords of the gonadal ridge and degeneration of the cortical sex cords. In the deep medulla of the gonad, the sex cords hollow out to form the rete testis. In the fourth month of development, the medullary sex cords are solid horseshoe shaped structures, connected at their extremities to the rate testis. The cords consist of primitive sex cells and supportive cells, Sertoli cells. These cords remain solid until puberty when they hollow out to become the seminiferous tubules. Some of the mesenchymal cells of the gonadal ridge differentiate into Leidig (interstitial) cells which begin to produce testosterone by the eighth week of gestation.
The gonads descend from their origin on the posterior body wall to their functional locations in both males and females. Each developing gonad is attached at its inferior pole to a fibromuscular band of tissue called the gubernaculum. The distal end of the gubernaculum is anchored in subcutaneous tissue of the labioscrotal swelling after passing through the inguinal canal. It guides the testis itoward the inguinal canal along with the attendant ductus deferens and blood vessels. As these structures pass through the inguinal canal, they acquire coverings derived from the anterolateral abdominal wall muscles.
- transversalis fascia - forms the deep inguinal ring and internal spermatic fascia
- transverses abdominis - no derivative
- internal abdominal oblique - cremaster muscle
- external abdominal oblique - superficial inguinal ring and external spermatic fascia
Although the ovary does not pass through the inguinal canal normally, the gubernaculum does, forming the round ligament of the ovary and the proper ovarian ligament.
Development of the Genital Ducts
Indifferent Stage
Initially, both males and females possess two sets of genital ducts. The mesonephric (Wolffian) ducts are principally associated with the mesonephric kidney. A second set of ducts develops anterior-lateral to the mesonephric ducts by invagination of the epithelium of the urogenital ridge. These paramesonephric (Mullerian) ducts cross the mesonephric ducts in their inferior course to the pelvis. The right and left paramesonephric ducts are open to the abdominal cavity at their cranial ends and come together to fuse at their caudal ends. Caudally, the combined paramesonephric ducts project into the wall of the urogenital sinus as the sinus tubercle. The mesonephric ducts open into the urogenital sinus on either side of the sinus tubercle.
Mesonephric Ducts
Development of the Male Duct System
In genetic males, the production of testosterone by the developing Leidig cells of the testis enhances the development of the mesonephric duct system. The original mesonephric tubules differentiate into the efferent ductules, which connect to the epididymis, formed from the convoluted elongation of the proximal end of the mesonephric duct.
The mesonephric duct distal to the epididymis forms the ductus deferens. Distally, an outgrowth of the mesonephric duct forms the primordium of the seminal vesicle and the portion of the mesonephric duct between the seminal vesicle and urogenital sinus is the ejaculatory duct. Sertoli cells, developing in the medullary sex cords of the testis secrete a substance called anti-Mullerian hormone or Mullerian inhibiting substance which promotes regression of the paramesonepric duct system in males. The exception of this is the distal part of the fused paramesonephric ducts that open into the presumptive prostatic urethra for form the prostatic utricle. The prostatic utricle is sometimes referred to as the “male uterus” due to its embryonic origin from the paramesonephric ducts.
Paramesonephric Ducts
Development of the Female Duct System
The presence of estrogen from maternal and placental sources promotes the development of the paramesonephric duct system for form the female genital tract. The absence of testosterone results in the regression of the mesonephric duct system. The cranial ends of the paramesonephric ducts remain open to the abdominal cavity and become the uterine tubes. The fused caudal ends of the paramesonephric ducts from the uterus and where they are attached by the sinus tubercle to the urogenital sinus, form the upper 1/3 of the vagina. The presence of the sinus tubercle causes proliferation of tissue from the posterior wall of the urogenital sinus for form the sinovaginal bulbs or vaginal plate. This tissue eventually develops a lumen to form the inferior 2/3 of the vagina. The phallic part of the urogenital sinus forms the vestibule of the vagina.
In this sagittal view of the developing female pelvis, components of the reproductive system derived from the fused paramesonephric ducts are indicated in brown and include the uterus and the upper third of the vagina. The sinovaginal bulbs (yellow) give rise to the lower two-thirds of the vagina. Initially, the vagina is separated from the vestibule by a membranous partition called the hymen. This membrane usually ruptures in the perinatal period.
Note the green line that represents the cut edge of the peritoneum. Observe the peritoneal relationships between uterus and rectum (rectouterine pouch) and uterus and bladder. (uterovesical pouch). Initially, the vagina is separated from the vestibule by a membranous partition called the hymen. This membrane usually ruptures in the perinatal period.
Development of the External Genitalia
Indifferent Stage
In the third week of development, mesenchyme migrates from the region of the primitive streak to surround the cloaca and produce the cloacal folds. Fusion of these folds anteriorly forms the genital tubercle. Caudally, the cloacal folds become separated into urethral folds and anal folds. The anal folds surround the anus and the mesoderm forms the anal sphincter. The urethral folds (aka urogenital folds) are reinforced laterally (buttress). Lateral to the urethral folds, a pair of labioscrotal swellings appears.
Female External Genitalia
| The genital |
Male External Genitalia
Androgens secreted by the fetal testes guide the development of male external genitalia. Elongation of the genital tubercle into a primordial phallus, pulls the urethral folds toward the midline. The urethral folds fuse to form the corpus spongiosum, enclosing the penile urethra while the genital tubercle contributes to the glans and body of the penis. The labioscrotal swellings move toward each other to fuse in the midline to form the scrotum. Eventually, the testis will descend through the inguinal canal to occupy the scrotum just prior to birth.