Gametes and their predecessor cells (the primordial germ cells) are differentiated from other somatic cells very early, and emigrate from the ectoderm (third week) via the extraembryonic endoderm (fifth week) into the primordium of the future gonads, the gonadal ridge. There, through an interaction with the coelomic epithelial cells, the primordium for the testis evolves in the seventh week, if a Y chromosome is present, or the primordium for the ovary in the eighth week, if it is absent.
The development of the testis occurs under the influence of testosterone among other factors. This is produced by the Leydig's interstitial cells that stem from the mesenchyma of the gonadal ridge, in an initial stage of activity (beginning of the 7th week). A second surge of secretory activity of the same cells starts at puberty. This leads to the maturation of the gonadal epithelium and to the growth and lumen formation in the tubuli seminiferi contorti.
Spermatogenesis which takes place from puberty onwards leads to a 64 day-long cycle in which the spermatogonia develop into sperm cells. At the beginning of spermatogenesis three steps of mitosis up to primary spermatocytes type I occur before meiosis commences. The first meiosis lasts 24 days, of which the prophase, with its four typical histological phases, takes the longest time. The secondary spermatocytes are engendered in the first meiosis and they immediately continue with the second meiosis, which is very brief because neither a synthesis of DNA nor a new grouping of the chromosomes takes place. The results of the second meiosis are the haploid spermatids. Within 24 days they differentiate themselves to become sperm cells that are then released into the lumen of the tubuli. Sperm cell production happens within innumerable temporally and spatially separated spermatogenesis waves that are spread throughout the whole lengths of the tubuli, that are wound up in each other in a spiral fashion. The sperm cell production is subject to large variations with an average value of around 100 million / day.
Oogenesis begins in roughly the 7th week (stage 20). The secondary germinal cords that have grown into the ovarian cortex decompose into individual groups of cells. A lively proliferation result, whereby the oogonia, similar to the spermatogonia, remain connected with each other via cellular bridges, permitting a synchronization of the mitosis and the subsequent meiosis steps (prophase). As soon as these oogonia have commenced with meiosis, they are named primary oocytes (12th week). All oocytes are arrested in the first meiosis at the end of the prophase. This interphase is termed dictyotene and can last until adulthood. The primary oocytes loose themselves from their cellular binding; they become surrounded by flat, somatic cells (follicle or granulosa cells) and are now called primordial follicles. In the 20th week nearly 7 million germ cells are formed and the whole cortex consists of these primordial follicles. Because primordial follicles can be dormant for up to 50 years in the human, the length of the ovarian cycle does not include this time. The supply of follicles decreases slightly before birth (2 million), and to 500,000 by puberty for the average case (populations at puberty range from 25,000 to 1.5 million).