Answer
Primordial germ cells form in the yolk sac, a membrane associated with the embryo (extra embryonic membrane). At the 5th-6th week of pregnancy these gem cells migrate into the embryo and inhabit the gonadal ridges.. There they develop into stem cells called spermatogonia. They occupy positions along the sides of the seminiferous tubules where they are outside the blood testis barrier (BTB). At birth the spermatogonia are still dormant and they remain dormant until the male child reaches puberty; then an increase in testosterone secretion initiates spermatogenesis.
Work Step by Step
When testosterone triggers spermatogenesis the spermatogonia divide by mitosis. Each activated cell divides into two daughter cells-- a Type A spermatogonium and a Type B spermatogonium. Type spermatogonia remain near the tube wall, and as a stem cells constitute the males lifetime supply of spermatogonia that keep the male fertile throughout life. However, the Type B spermatogonia migrate away from the tube wall, enlarges and differentiate into a primary (2n) spermatocytes.
