Label retaining cells and cutaneous stem cells.

This is a comprehensive review on label retaining cells (LRC) in epidermal development and homeostasis. The precise in vivo identification and location of epidermal stem cells is a crucial issue in cutaneous biology. We discuss here the following problems: (1) Identification and location of LRC in the interfollicular epithelium and hair follicle; (2) The proliferative potential of LRC and their role in cutaneous homeostasis (3); LRC phenomenon and the Immortal Strand Hypothesis, which suggests an alternative mechanism for retention of genetic information; (4) Significance of LRC studies for development of stem cell concept. Now, it seems evident that LRC are a frequent feature of stem cell niches and revealing highly dormant LRC may be used for identification of stem cell niches in different tissues. LRC were used for screening specific markers of epidermal stem cells. Within a given tissue stem cells have different proliferative characteristics. There are more frequently cycling stem cells which function primarily in homeostasis, while LRC form a reserve of dormant, may be ultimate, stem cells, which are set aside for regeneration of injury or unforeseen need. The authors suggest that LRC dormancy described in Mammalia has much in common with developmental quiescence found in some other animals. For example in C. elegans reproductive system, vulval precursor cells have developmentally programmed cell-cycle arrest in the first larval stage, and then undergo an extended period of quiescence before resuming proliferation. Another example of developmental quiescence is the diapause, a widespread phenomenon exhibited by animals ranging from nematodes to mammals, often occurring at genetically predetermined life history stage.