From worm to germ: Germ cell development and regeneration in planarians.

The specification and proper differentiation of germ cells ensure the propagation of sexually reproducing species. Studies of a wide range of organisms have uncovered several important, conserved features of germ cell development, including the critical roles played by localized niches and somatically derived systemic cues. The planarian Schmidtea mediterranea is an excellent model to study fundamental aspects of germ cell development. Planarians are well known for their remarkable regenerative abilities and can regenerate whole animals from small tissue fragments. This amazing ability is bestowed by neoblasts, pluripotent somatic stem cells that are maintained throughout the planarian's lifetime. Advances in functional genomic methodologies have made planarians a powerful model to investigate the molecular mechanisms underlying germ cell development and reproductive maturation. Here we review recent studies that have led to the discovery of several germ cell-intrinsic factors and somatically derived extrinsic signals important for regulating various aspects of germ cell development. In addition to revealing deep conservation of mechanisms that intrinsically regulate germ cells, these studies also uncover an important function for neuroendocrine control of planarian reproduction as well as novel roles for GPCR signaling in the somatic gonadal niche.