The embryology of the male and the female reproductive systems:
By the sixth week of development in both sexes, the primordial germ cell have migrated from their earlier extra embryonic location to the gonadal ridges, where they are surrounded by the sex cords to form a pair of primitive gonads. Up to this time, the developing gonad, whether chromosomally XX or XY, is bipotential.
The current concept is that development into an ovary or a testis is determined by the coordinated action of a sequence of genes that lead to ovarian development when no Y chromosome is present or to testicular development if a Y is present. The ovarian pathway is followed unless a gene on the short arm of the Y, designated TDF (testis-determined factor), acts as a switch, diverting development into the male pathway. The search for the major testis-determined gene is one of the leading current problems in medical genetics.
In the presence of Y Chromosome, the medullary tissue forms typical testes with seminiferous tubules and Leydig cells, which, under the stimululation of human chorionic gonadotropin from the placenta, become capable of androgen secretion. The spermatogonia, derived from the primordial germ cells by 200 or more successive mitosis, form the walls of the semiferous tubules together with supporting Sertoli cells.
If no Y chromosome is present, the gonad, by default, forms an ovary; the cortex develops, the medulla regresses, and oogonia begin to develop within follicles. The oogonia are derived from the primitive germ cells by a series of about 30 mitoses, for fewer than the number required for spermatogenesis. Beginning at about the end of the third month, the oogonia enter meiosis I, but this this process is arrested at a stage called dictyotene, in which the cell remains until ovulation occurs many years later. Many of the oogonia degenerate before birth, and only about 400 mature into ova during the 30 years or so of sexual maturity of the female.
While the primordial germ cells are migrating to the genital ridges, thickenings in the ridges indicate the developing genital ducts, the mesonephric (formerly called Wolffian) and paramesonephric (formely called Mullerian) ducts. In the male, the Leydig cells of the fetal testes produce androgen, which stimulates the mesonephric ducts to form the male genital ducts, and the Sertoli cells produce a hormone that suppresses formation of the paramesonephric ducts. In the female (or in an embryo with no gonads), the mesonephric ducts regress, and the paramesonephric ducts develop into the female duct system. In the early embryo, the external genitalia consist of a genital tubercle, paired labioscrotal swellings, and paired urethral folds. From this undifferentiated state, male external genitalia develop under the influence of androgens, or, in the absence of a testis, female external genitalia are formed regardless of whether an ovary is present.