Cutaneous stem cells: something new and something borrowed.

Stem cells are cells that, upon division, produce new stem cells as well as daughter cells that initiate differentiation along a specific lineage. The epidermis is a rapidly self-renewing tissue. The epidermis maintains homeostasis by constant proliferation of the basal layer of rapidly dividing progeny of stem cells-epidermal stem cells. However, the diversity between epithelial functions in different organs makes it difficult to determine whether common properties exist in regulating these related epidermal stem cells. Recently, studies have confirmed that adult stem cells reside in specific niches. Signals emanating from the niche regulate stem cell self-renewal, survival, and maintenance; surrounding support microenvironment and cell-cell adhesion between stem cells and surrounding support cells also contribute to these cells' remarkable behavior. In addition, a combination of localized signaling pathways and autonomously acting proteins has proved to contribute to skin stem cells' proliferation and asymmetric divisions. In this paper, we focus on comparing several kinds of stem cells in the epidermis, signaling influences of the stem cell and its niche in actively self-renewing epithelia, and introducing something new and something borrowed.