Germline stem cells: stems of the next generation.

Germline stem cells (GSCs) sustain gametogenesis during the life of organisms. Recent progress has substantially extended our understanding of GSC behavior, including the mechanisms of stem cell self-renewal, asymmetric stem cell division, stem cell niches, dedifferentiation, and tissue aging. GSCs typically are highly proliferative, owing to organismal requirement to produce large number of differentiated cells. While many somatic stem cells are multipotent, with multiple differentiation pathways, GSCs are unipotent. For these relatively simple characteristics (e.g. constant proliferation and unipotency), GSCs have served as ideal model systems for the study of adult stem cell behavior, leading to many important discoveries. Here, we summarize recent progress in GSC biology, with an emphasis on evolutionarily conserved mechanisms.