primordial germ cell

Primordial germ cells are the earliest precursors to gametes (sperm and egg cells). These cells are distinct from somatic (body) cells and are often segregated very early in embryonic development, sometimes even before the embryo divides into germ layers.12

In some invertebrates, the very first cell divisions can already identify a primordial germ cell. In vertebrates, these cells originate outside the developing gonads (ovaries or testes) and migrate to them. For instance, in mammals, fewer than 100 primary germ cells can increase to over 5,000 during their migration into the gonads.1

While primordial germ cells give rise to the actual gametes, the supporting cells within the gonads are derived from the embryo's mesoderm. The process of forming egg cells is called oogenesis, and the formation of sperm cells is called spermatogenesis. A primordial ovum is one of these large cells found in embryos from which true eggs are derived.123