Gonocytes, the forgotten cells of the germ cell lineage.

Male germ cells, the repository cells of the genome, comprise several successive developmental stages starting in the embryo and ending up with the spermatozoon. Gonocytes represent the fetal and neonatal stages preceding the formation of spermatogonial stem cells. Recent findings shows that germline stem cells can be driven to pluripotency and used as alternative for embryonic stem cells prompted more effort in identifying the processes regulating the development of their precursors, the testicular gonocytes. Also called pre- or pro-spermatogonia, gonocytes represent not one, but several successive developmental stages between the time at which the germ cell becomes resident in the forming fetal testis to the time it migrates to the basement membrane of the seminiferous cord to adopt a spermatogonial phenotype. This review summarizes the findings regarding the genetic identity of gonocytes, providing a description of the "common" gene expression profiles of fetal and neonatal gonocytes, as well as information on the main regulatory factors of gonocyte functions. A better comprehension of gonocyte development should help in the understanding of how germline stem cells are formed, possibly providing valuable clues on the origins of germ cell tumors or infertility.