In species with a short gestation, development of fetal germ cells occurs in a synchronous manner. For example, in mice, PGCs within the testis cords differentiate into a single population of fetal germ cells, usually referred to as ‘‘gonocytes,’’ that actively proliferate until about Day 15.5 p.c. and then remain arrested in Gi until Day 1.5 postpartum, when they resume mitotic activity. Two days before they resume reproliferation, the gonocytes relocate to the periphery of the cord, make contact with the basement membrane, and thereafter, undergo further differentiation via a process that is as yet poorly understood.
In humans, the fetal germ cell population in the testis is variously referred to as consisting of ‘‘gonocytes’’ or “prespermatogonia’’ and is often assumed to represent a homogenous population, as in rodents. This view is supported by the expression of proteins such as DAZL, VASA, and CHK2, which are reported to be present in all germ cells during the second trimester. However, histological evaluations conducted using electron microscopy more than 20 yr ago recorded the existence of distinct subpopulations of human fetal germ cells based on their morphological appearance.