HUMAN SEXUALITY: MORPHOLOGIC DIFFERENTIATION OF THE GENITALIA
The internal genitalia differentiate ahead of the external genitalia. Prenatal hormones, their presence or absence, control the differentiation of both.
The male and female internal genital structures differentiate from two separate primordia, the wolffian and mullerian ducts, respectively. In normal male development, the fetal testes first secrete their two hormones, androgen and mullerian inhibiting substance (MIS) at around the sixth week of gestation. Androgen ensures that the wolffian ducts proliferate into the vas deferens, seminal vesicles, and the ejaculatory ducts of the male internal genitalia. MIS ensures that the mullerian ducts vestigiate. In the absence of MIS, the mullerian ducts proliferate into a uterus, fallopian tubes, and the upper vagina of the female internal genitalia. In normal female development, the fetal ovaries do not, so far as is known, secrete hormones prenatally, and if maternal or placental hormones play a role in female differentiation, it has yet to be demonstrated.
Whereas the male and female internal genital structures differentiate from two separate primordia, the external genitalia develop from the same primordia. These structures remain undifferentiated until approximately the eighth week of fetal life, at which time androgen secreted from the fetal testes begins to masculinize them. It is possible that testosterone is converted to 5?-dihydrotestosterone (DHT) before masculinization of the external genitalia is effected. In any event, in the presence of androgen, the genital tubercle differentiates into a penis. The skin that becomes the hood of the clitoris and labia minora in the female wraps around the penis to cover it and to form the foreskin. The labioscrotal swellings fuse and form the scrotum. In the absence of androgen, the genital tubercle remains small and becomes the clitoris. What in the male is the foreskin and outer covering of the penis becomes the hood of the clitoris and the labia minora in the female. The labioscrotal swellings remain separate and form the labia majora.
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