Sexual differentiation in humans

It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination.

[1] Sexual differentiation includes development of different genitalia and the internal genital tracts and body hair plays a role in sex identification.

The presence of the SRY gene on the Y chromosome causes the development of the testes in males, and the subsequent release of hormones which cause the paramesonephric ducts to regress.

Disorders of sexual development (DSD), encompassing conditions characterized by the appearance of undeveloped genitals that may be ambiguous, or look like those typical for the opposite sex, sometimes known as intersex, can be a result of genetic and hormonal factors.

[4] Most mammals, including humans, have an XY sex-determination system: the Y chromosome carries factors responsible for triggering male development.

[6] Six weeks elapse after fertilization before the first signs of sex differentiation can be observed in human embryos.

Over the next several weeks, hormones are produced that cause undifferentiated tissue to transform into either male or female reproductive organs.

Male development can only occur when the fetal testis secretes key hormones at a critical period in early gestation.

Testosterone converts the mesonephric ducts into male accessory structures, including the epididymides, vasa deferentia, and seminal vesicles.

Males become externally distinct between 8 and 12 weeks, as androgens enlarge the genital tubercle and cause the urogenital groove and sinus to fuse in the midline, producing an unambiguous penis with a phallic urethra, and the labioscrotal swellings become a thinned, rugate scrotum where the testicles are situated.

In some diseases and circumstances, other androgens may be present in high enough concentrations to cause partial or (rarely) complete masculinization of the external genitalia of a genetically female fetus.

Testosterone, which is secreted and converts the mesonephric ducts into male accessory structures, such as epididymis, vas deferens and seminal vesicle.

Male levels of testosterone directly induce growth of the penis, and indirectly (via DHT) the prostate.

This was later determined to be paramesonephric inhibiting substance (MIS), a 140 kD dimeric glycoprotein that is produced by Sertoli cells.

[12] Visible differentiation occurs at puberty, when estradiol and other hormones cause breasts to develop in typical females.