ABSTRACT
Sexual differentiation in humans is genetically and hormonally controlled. In response
to a signal from a dominant-acting gene on the Y chromosome, primordial cells in the
embryonic gonad ridge differentiate into Sertoli cells and affect newly migrated germ
cells to differentiate as spermatogonia, thus creating a testis. The cells of the
embryonic testis secrete hormones that lead to the development of most, if not all,
male secondary sexual characteristics. The Sertoli cells secrete müllerian inhibitory
factor (MIF), causing regression of the müllerian ducts and of stray oogonia. The
Leydig cells secrete testosterone, causing differentiation and growth of the wolffian
duct structures. Dihydrotestosterone, created by metabolism of testosterone, causes
growth of the prostate and phallus and fusion of the labioscrotal folds. In the absence
of SRY, Sertoli cell differentiation does not occur. Rather germ cells migrating into the
primordial gonad differentiate as oogonia and cause interstitial cells to differentiate
as granulosa cells. In the absence of MIF and testosterone, the müllerian ducts differentiate
and grow as female internal genitalia and the external genitalia are feminized. Several
genes have been identified that control testis determination. These include SRY, WT1, SOX9, SF1, XH2, and DAX1. Most of these genes were discovered by analysis of rare cases of sex reversal (genetic
sex of one type, gonadal sex of the other type).
KEYWORD
determination - gonadal differentiation - intersex conditions - gonadal dysgenesis
- hermaphroditism
