Abstract
Mouse spermatogenesis is supported by spermatogenic stem cells (SSCs). SSCs maintain their pool while migrating over an open (or facultative) niche microenvironment of testicular seminiferous tubules, where ligands that support self-renewal are likely distributed widely. This contrasts with the classic picture of closed (or definitive) niches in which stem cells are gathered and the ligands are highly localized. Some of the key properties observed in the dynamics of SSCs in the testicular niche in vivo, which show the flexible and stochastic (probabilistic) fate behaviors, are found to be generic for a wide range of, if not all, tissue stem cells. SSCs also show properties characteristic of an open niche-supported system, such as high motility. Motivated by the properties of SSCs, in this review, I will reconsider the potential unity and diversity of tissue stem cell systems, with an emphasis on the varying degrees of ligand distribution and stem cell motility.