Extracellular membrane markers of fetal male gonocytes. Gonocytes also co-expressed C-Kit (A) and THY1 (B), shown at 15 and 19 weeks of gestation, respectively. While SSEA-4+ is a general marker for all germ and Sertoli cells in the fetal testes, THY1 expression within the SSEA-4 population was restricted only to gonocytes that expressed low level of VASA (VASAD) (C) and OCT4A (D), shown at 21 and 18 weeks of gestation, respectively. Arrow heads indicate VASAD cells (gonocytes). Donkey anti-goat Alexa 488 (VASA), anti-mouse Alexa 488 (C-Kit), anti-rabbit Alexa 594 (OCT4A), and anti-mouse Alexa 555 (THY1) were used.