Germ-Cell Specification in Xenopus

Primordial germ cells (PGCs) represent the only precursor stem cell population that normally gives rise to the gametes. For this reason, PGCs have been called the "stem cells of the species" and are prime examples of a normal totipotent state (Wylie, 1999. Cell 96 (2): 165-174). PGCs represent a remarkable lineage that will differentiate into highly specialized cells, either oocytes or sperm, yet also retain the ability to generate a complete new individual. Thus, a fundamental question in developmental and stem cell biology is how PGCs differentiate while retaining the potential for totipotency in contrast to somatic cells that become fate-restricted. In this chapter, we will review what is known about how Xenopus sets aside its germ-cell lineage.