Step 1: Sex Chromosomal Complement
The first step to typical sex differentiation occurs when a mom’s egg is fertilized by a dad’s sperm. Mom’s egg contains an X chromosome that she shares with her developing baby. Dad’s sperm contains either an X chromosome or a Y chromosome that he shares with his developing baby.
Step 2: Development of Reproductive Tissues Common to Both XY and XX Fetuses
The second step to typical sex differentiation occurs when cells organize to form tissues. Important for whether a fetus develops into a boy or a girl are the following common tissues: (1) gonadal ridges, (2) internal sex ducts and (3) external genital tissue.
During the first 2 months of pregnancy the gonadal ridges form. These ridges are identical in XY and XX fetuses. As fetal development continues past 2 months, these ridges will differentiate into testes for boys and ovaries for girls.
During the 2nd month of pregnancy all fetuses (XX and XY) have 2 sets of internal sex ducts – a male set (prostate gland, vas deferens, seminal vesicles) called Wolffian ducts and a female set (cervix, uterus, fallopian tubes) called Mullerian ducts. As fetal development continues, XY fetuses will keep their Wolffian ducts and lose their Mullerian ducts. Conversely, XX fetuses will keep their Mullerian ducts and lose their Wolffian ducts.
Step 3: Differentiation of Gonadal Ridges into Testes or Ovaries
The third step to typical sex differentiation occurs when the gonadal ridges develop into either testes or ovaries. This 3rd step is under the control of our genes. In XY fetuses, a gene on the Y chromosome called SRY triggers the gonadal ridges to develop into testes. We do not fully understand at this time what triggers the gonadal ridges to develop into ovaries in fetuses that do not have an SRY gene.
Step 4: Differentiation of Common Internal Sex Ducts into Male or Female Sex Ducts
The fourth step to typical sex differentiation occurs when common sex ducts develop into either a male-typical or a female-typical pattern. This 4th step is under the control of hormones. In XY fetuses, the developing testes secrete 2 hormones, testosterone and Mullerian Inhibiting Substance (MIS). Testosterone allows the Wolffian ducts to continue to develop while MIS causes the Mullerian ducts to disappear. The result of these hormonal actions is a fetus with male (Wolffian) ducts but no female (Mullerian) ducts.
In XX fetuses, the developing ovaries do not secrete hormones. Without testosterone the Wolffian ducts disappear, and without MIS the Mullerian ducts continue to develop. The result of not being exposed to testicular hormones during prenatal development is a fetus with female (Mullerian) ducts but no male (Wolffian) ducts.
Step 5: Differentiation of Common External Genital Tissue into Male or Female External Genitalia
The 5th step to typical sex differentiation occurs when female-appearing external genital tissue develops and continues to appear female or develops into male-typical external genitalia. This 5 th step, like the 4 th step, is under hormonal control.
In XY fetuses with testes that produce testosterone, the testosterone is converted to a stronger form of the hormone called dihydrotestosterone (DHT). After the female-appearing external genitalia of the XY fetus is exposed to DHT, it grows and changes until it appears male.
When all 5 of these steps proceed as either male-typical or female-typical development, a boy or girl is born, respectively.