Physiology, Testosterone

Excerpt

Testosterone is the primary male hormone regulating sex differentiation, producing male sex characteristics, spermatogenesis, and fertility. Testosterone’s effects are first seen in the fetus. During the first 6 weeks of development, the reproductive tissues of males and females are identical. Around week 7 in utero, the SRY (sex-related gene on the Y chromosome) initiates the development of the testicles. Sertoli cells from the testis cords (fetal testicles) eventually develop into seminiferous tubules. Sertoli cells produce a Mullerian-inhibiting substance (MIS), which leads to the regression of the Fallopian tubes, uterus, and upper segment of the vagina (Mullerian structures normally present in females). Fetal Leydig and endothelial cells migrate into the gonad and produce testosterone, which supports the differentiation of the Wolffian duct (mesonephric duct) structures that become the male urogenital tract. Testosterone also gets converted to dihydrotestosterone (DHT) in the periphery (discussed below) and induces the formation of the prostate and male external genitalia. Testosterone is also responsible for testicular descent through the inguinal canal, which occurs in the last 2 months of fetal development. When an embryo lacks a Y chromosome and thus the SRY gene, ovaries develop. Fetal ovaries do not produce adequate amounts of testosterone; thus, the Wolffian ducts do not develop. There is also an absence of MIS in these individuals, leading to the development of the Mullerian ducts and female reproductive structures.