9 Primordial Germ Cells as Stem Cells

Abstract

Germ cells are the precursors of the mature gametes, making their status as stem cells apparently unassailable. The fusion of the gametes to produce a totipotent zygote initiates the whole program of embryonic development, leading to the formation of the stem cells of all adult tissues as well as the next generation of germ cells. Focusing on mammals, in this review, I examine how germ cells arise and whether their precursors are stem cells in their own right. I also discuss how the study of germ cells and their precursors sheds light on the important questions of what controls pluripotency and how genomes are reprogrammed. For the purposes of this review, I define stem cells as a cell population that has the capacity both to self-renew and to give rise to at least one kind of nondividing, fully differentiated descendant.

The germ cell lineage usually originates as a very small founding population that is segregated from somatic cells early in development, at least in organisms where the overall body plan is also established early (Dixon 1994). Perhaps the physical separation of germ cells from organizing centers helps to protect them from the influence of potent signaling factors and morphogenetic movements. In vertebrates and Drosophila, there is considerable proliferation of the founding population as it moves from its site of origin to the gonads. The term primordial germ cells (PGCs) is strictly applied to the diploid germ cell precursors that transiently exist in the embryo before they enter into close association...