The genital ridges appear as thickenings of the intermediate mesoderm at around 4-5 wk of gestation in humans and 11-11.5 days postcoitum (p.c.) in mice. Within the genital ridges, functional differentiation of Sertoli cells from somatic precursors is dependent on expression of the SRY gene as well as other gene products, including WT-1 and SOX-9. The first morphological sign of testicular differentiation is the formation of testicular cords; this occurs gradually in humans (between 6 and 8 wk ) but in a matter of hours in rodents (completed by 12.5-13.5 days p.c. in mice).
In mice, a population of proliferating primordial germ cells (PGCs), identified by staining for alkaline phosphatase, migrate via the embryonic ectoderm to reach the gonadal ridge around 11 days p.c.. In human, PGCs start migrating at 4 wk of gestation and enter the gonad during the fifth week (for review, see ). Once they reach the genital ridge, the PGCs become enclosed by the differentiating Sertoli cells so that they reside within the testis cords; this occurs on or about the seventh week of pregnancy in humans.