Stem cell competition for niche occupancy: emerging themes and mechanisms.

Tissue-specific adult stem cells are responsible for generating a range of various differentiated cells during their life time; these cells are intimately associated with niche for maintenance and function. Recent studies of germline stem cell niches in Drosophila gonad suggest that stem cells within a niche constantly compete with each other for niche occupancy. Competition within a niche occurs between same type of stem cells as well as different types. In both cases, cell adhesion molecules are critical in mediating the competition.