Gametogonium

Gametogonium (plural gametogonia) are stem cells for gametes located within the gonads. They originate from primordial germ cells, which have migrated to the gonads. Male gametogonia which are located within the testes during development and adulthood are called spermatogonium (plural spermatogonia). Female gametogonia, known as oogonium (plural oogonia), are found within the ovaries of the developing foetus and were thought to be depleted at or after birth. Spermatogonia and oogonia are classified as sexually differentiated germ cells.

Germ cells are specified early in development and can only differentiate into gametes. The segregation of germ cells is often determined by the species, with some undergoing preformation, where the germ cells are determined by maternally inherited factors before or immediately after fertilisation, and others undergoing epigenesis, where the germ cell lineage is determined from signalling from surrounding tissues. Preformation was initially perceived as more common than epigenesis, as it appears in many model organisms like the common fruit fly, roundworms and some amphibians. Epigenesis has since been shown to be the more common mechanism. The specific mechanism of germ line differentiation varies across species.

Mice and other mammalian species undergo epigenesis during development, where germ cells are separated from the somatic lineage during early gastrulation, occurring at embryonic day 7 in mice, and are derived directly from proximal epiblast cells relative to the extraembryonic ectoderm. Prior to gastrulation the epiblast cells are not yet set in their role as cells of the germ lineage and can act as precursors for somatic cells Matsui and Okamura, 2003. At this stage, cells transplanted to the proximal epiblast from other parts of the epiblast can also be differentiated into germ line cells. The potential germ line cells are specified by the extracellular signalling of BMP4, BMP2 and BMP8b from the extraembryonic ectoderm. The germ cell population (~40 in mice), after specification, migrate to the developing gonads where they differentiate further into gametogonia. Much of the research in germ cell development is done on animal models. Animal models are an effective research tool due the commonality of sexual reproduction which is thought to have same or similar mechanisms across species. The majority of research is done on mice which has led to advances in understanding germ line differentiation across all mammal but there are some species specific mechanisms which have not been studied as extensively due to the difficulty of both obtaining human samples and the ethical limitations of human research. To circumvent that, there have been studies performed on human pluripotent stem cells.

...
Wikipedia