Wednesday, February 11, 2015
Female Reproductive System Development
Development of Gonads
Endodermal cells, called primordial germ cells, migrate from the wall of the yolk sac near the allantois to the dorsal wall of the abdominal cavity during the third week. The primordial germ cells enter the gonadal ridges that parallel the mesonephros. Each ridge has a thick epithelium continuous with columns of cells called primary sex cords that extend into the center (medulla) of the ridge. At this gender indifferent stage, male embryos are indistinguishable from female embryos.
Development of the Ovaries
The primary sex cords in the female embryo degenerate and the primordial germ cells migrate into the outer region (cortex) of the genital ridge that will become the ovary.
Development of Ducts and Accessory Organs
A paramesonephric (Müllerian) duct develops anterior to each mesonephric duct in both sexes. The paramesonephric duct has no connection to the kidneys; instead it extends along the genital ridge and continues toward the cloaca. Thus, both genders have mesonephric and paramesonephric ducts during the indifferent stage of sexual development.
The embryo—regardless of its genetic sex—will develop into a female unless exposed to androgens. In a normal male embryo, cells in the medulla of the gonadal ridge begin to produce testosterone sometime after the sixth week. Testosterone triggers changes in the duct system and external genitalia shown in these figures.
Development of Genital Ducts in the Female
The mesonephric tubules and duct degenerate in the female embryo. The paramesonephric ducts develop a broad proximal opening into the peritoneal cavity; they fuse distally to form the uterus.
Development of External Genitalia
After four weeks of development, mesenchymal swellings called cloacal folds develop around the cloacal membrane. A genital tubercle develops that will form the glans of the penis in males and the clitoris in females. At six weeks, the cloaca has been subdivided, separating the cloacal membrane into a posterior anal membrane bounded by anal folds, and an anterior urogenital membrane bounded by the urethral folds. A prominent genital swelling develops lateral to each urethral fold.
Development of Female External Genitalia
In the female, the urethral folds do not fuse but develop into the labia minora. The genital swellings will become the labia majora. The genital tubercle develops into theclitoris, immediately posterior to which the urethra opens to the exterior. The hymen persists as an elaboration of the urogenital membrane.