Sexual differentiation refers to the embryological development of male and female phenotypes. Unlike sexual genotype which is determined at the time of fertilisation, the male and female phenotypes do not begin to differentiate substantially until the seventh week of gestation.
Males
Y chromosome expresses the SRY protein (Sex-determining Region on Y) which promotes activation of many genes involved in patterning of the male phenotype.
The testes form from the genital ridges, a pair of mesenchymal condensates on the posterior abdominal wall, either side of the midline and anterior to the mesonephri, or "middle kidneys".
The mesonephri will eventually regress, and their connecting mesonephric (Wolffian) ducts will become the excretory ducts of the testes (ductus deferens) under the presence of testosterone.
Simultaneously, a second pair of ducts arises from an invagination of the overlying peritoneum and run adjacent to the Wolffian ducts. These are the paramesonephric ducts, also known as the Mullerian ducts. In the male, testicular secretions of anti-Mullerian hormone result in their regression.
Regression of the mesonephri leaves the developing gonads suspended on a mesentery, known as the mesorchium in males, and mesovarium in females).
The genitoinguinal ligament connects the caudal end of the gonad to the inguinal opening. INSL-3 and androgens facilitate the descent of the testes into the scrotum.
Females
A lack of anti-Mullerian hormone prevents the regression of the mesonephric ducts which develop into the fallopian tubes and upper vagina. The gonads (ovaries) are suspended on a mesentery known as the mesovarium.
A lack of high concentration of testosterone results in regression of the Wolffian ducts.
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