In the female embryo, the primary sex cords give rise to the medullary cords, which persist only for a short time. The coelomic epithelium again proliferates into the underlying mesenchyme, and these strands are the cortical cords. By the fourth month, the cortical cords begin to form isolated cell clusters called primordial follicles. These follicles contain the oogonia, which derive from primordial germ cells and are surrounded by a single layer of lattened follicular cells derived from the cortical cords. Follicular cells serve as supporting nutrient cells. By 8 months, the ovary has become a long, narrow, lobulated structure that is attached to the body wall by the mesovarium. The coelomic epithelium has been separated by a band of connective tissue-tunica albuginea-from the cortex. At this stage, the cortex contains follicles and is well deined from the inner medulla, which is composed of abundant blood vessels, lymphatic vessels, and nerve ibers.
• Embryology of the External Genitalia
Early development of the external genitalia is similar in both sexes. By 6 weeks' gestation, three external protuberances have developed surrounding the cloacal membrane. These are the left and right cloacal folds, which meet ventrally to form the genital tubercle (Fig. 3-4). With division of the cloacal membrane into anal and urogenital membranes, the cloacal folds become the anal and urethral folds, respectively. Lateral to the urethral folds, genital swellings arise, and these become the labioscrotal folds. Between the urethral folds, the urogenital sinus extends onto the surface of the enlarging genital tubercle to form the urethral groove. By week 7, the urogenital membrane ruptures, exposing the cavity of the urogenital sinus to amnionic fluid.
The genital tubercle elongates to form the phallus in males and the clitoris in females. Still, it is not possible to visually diferentiate between male and female external genitalia until week 12. In the male fetus, dihydrotestosterone (DHT) forms locally by the 5-a reduction of testosterone. DHT prompts the anogenital distance to lengthen, the phallus to enlarge, and the labioscrotal folds to fuse and form the scrotum.
In the female fetus, without DHT, the anogenital distance does not lengthen, and the labioscrotal and urethral folds do not fuse (Fig. 3-4C). The genital tubercle bends caudally to become the clitoris, and the urogenital sinus forms the vestibule of the vagina. The labioscrotal folds create the labia majora, whereas the urethral folds persist as the labia minora. Female external genital diferentiation is complete by 11 weeks, whereas male external genital diferentiation is complete by 14 weeks.
Degenerating paramesonephric duct Primordial germ cell Uterine tube -FIGURE 3-3 Embryonic gonad diferentiation. TDF Persistent paramesonephric duct Coelomic epithelium Degenerating medullary cords = testis-determining factor.
Labioscrotal/ ,
FIGURE 3-4 Development ofthe external genitalia. A. Indifferent stage. B. Virilization ofexternal genitalia. C. Feminization. (Reproduced with permission from Bradshaw KD: Anatomical disorders. In Hoffman BL, Schorge JO, Bradshaw KD, et al (eds): Williams Gynecology, 3rd ed. New York, McGraw Hill Education, 201o6.)
Defining gender incorporates genetic gender, gonadal gender, and phenotypic gender. Genetic gender-X or Y -is established at fertilization. However, for the irst 6 weeks, development of male and female embf) os is morphologically indistinguishable.