Tissue-specific designs of stem cell hierarchies.

Recent work in the field of stem cell biology suggests that there is no single design for an adult tissue stem cell hierarchy, and that different tissues employ distinct strategies to meet their self-renewal and repair requirements. Stem cells may be multipotent or unipotent, and can exist in quiescent or actively dividing states. 'Professional' stem cells may also co-exist with facultative stem cells, which are more specialized daughter cells that revert to a stem cell state under specific tissue damage conditions. Here, we discuss stem cell strategies as seen in three solid mammalian tissues: the intestine, mammary gland and skeletal muscle.