Reproductive System: Ducts And Genitalia
Time period: day 35 to postnatal development
The reproductive systems develop from a series of epithelial cell‐lined ducts, derived from mesoderm. The initial stage of genital development is the same for both sexes up to week 7, and is called the indifferent stage.
The indifferent stage involves the mesonephric ducts (or Wolffian ducts) from the developing urinary system and the paramesonephric ducts (or Müllerian ducts), named because of their location lateral to the mesonephric ducts (Figures 37.1 and 37.2). The paramesonephric ducts form from longitudinal invaginations of the surface epithelium of the gondal ridge.
The paramesonephric ducts descend, meet in the midline and fuse in the pelvic region to form the uterovaginal primordium (Figure 37.3). This bulges into the dorsal wall of the developing urogenital sinus (see Chapters 33 and 36) but does not break the wall. The bulge forms the paramesonephric tubercle (or sinus tubercle, or Müller tubercle).
The paramesonephric ducts open into the peritoneal cavity, and the free unfused cranial ends become the uterine tubes. The uterus forms from the midline uterovaginal primordium.
The paramesonephric tubercle induces the urogenital sinus to form 2 outgrowths of cells within its lumen. These outgrowths proliferate and form the sinovaginal bulbs, which fuse and form the vaginal plate (Figure 37.4). This will canalise to form a hollow core, which is completed by the fifth month.
The inferior part of the vagina probably forms from the vaginal plate, and the superior part from uterovaginal primordium. The vagina is separated from the urogenital sinus by the hymen.
The female reproductive system (Figure 37.5) is likely to grow from 2 tissue origins: the lining of the lower portion of the vagina is endodermal and the upper portion, fornices and uterus are meso- dermal. The muscle and connective tissues of the vagina and uterus are derived from the surrounding mesenchyme.
The mesonephric ducts degenerate, although remnants may remain.
Mesonephric ducts become the efferent ductules and epididymis of the testes, the ductus deferens (or vas deferens) and the ejaculatory duct (Figures 37.6 and 37.7).
The seminal vesicles form as an outgrowth from the ductus deferens, whereas the prostate gland arises from numerous out-growths from the urethra. The endodermal cells of the urethra differentiate to become the glandular tissue of the prostate gland, and the surrounding mesenchyme forms the smooth muscle and connective tissue.
Paramesonephric ducts degenerate (although remnants can remain).
Until the ninth week of development the external genitals appear the same for both sexes (Figures 37.8 and 37.9). You cannot see the difference in the sex of a developing embryo until around 11 weeks’ gestation. To prevent mistakes made in ultrasound identification, if the sex of the foetus is required it is identified at the 20‐week scan.
During the indifferent stage, the cloacal membrane is surrounded by mesenchymal folds called urogenital (cloacal) folds that fuse ventrally into a genital tubercle. Around week 7, the urogenital septum splits the cloacal membrane into a ventral urogenital membrane and a dorsal anal membrane.
Another pair of folds develop lateral to the urogenital folds, called the labioscrotal swellings. The urogenital membrane degenerates leaving the urogenital sinus in direct communication with the amniotic cavity. The genital tubercle elongates and forms the primordial phallus.
Induced by oestrogens secreted from the placenta and foetal ovaries, the genital turbercle develops into the clitoris (Figure 37.8). During the third and fourth months the clitoris is larger than its male counterpart. The urogenital groove remains open and develops into the vestibule which contains the openings of the vagina and urethra. The urogenital folds remain largely unfused (the two sides only meet posteriorly) and become the labia minora. The labioscrotal swellings become the labia majora.
Induced by androgens secreted from the developing testes, the primordial phallus grows to form the penis (Figure 37.9). The urogenital sinus forms a groove bound laterally by the urogenital folds, and endodermal cells divide and line the groove which is now termed the urethral plate. The urethral folds eventually fuse on the underside (penile raphe) surrounding a tube (the spongy part of the urethra).
The urethra temporarily ends blindly in the anterior part of the penis. In the fourth month the terminal part of the urethra is formed when cells from the glans grow internally producing an epithelial cord. A lumen then forms and creates the external urethral meatus. The lateral genital swellings form the scrotum and the visible line of fusion is the scrotal raphe.
The SRY gene (sex‐determining region of the Y chromosome) encodes for a transcription factor that is expressed in the gonad during the indifferent stage, triggering male development. If this transcription factor is absent female development occurs.
Hypospadias is caused by incomplete fusion of the urethral folds in the male, and the urethra opens onto the ventral surface of the penis. Epispadias results from the genital tubercle developing in the area of the urorectal septum, causing the urethra to open on the dorsal surface of the penis. Epispadias usually occurs in males but can occur in females and results in a split clitoris and an abnormally positioned urethral opening.
Congenital adrenal hyperplasia is an enzyme deficiency causing the adrenal glands to fail to produce sufficient cortisol and aldosterone, but the body produces excess androgens. This can result in ambiguous genitalia development in females but will not affect males. Further developmental problems occur, such as precocious puberty.