Chapter 15 - Epithelial Skin Stem Cells

Abstract

Skin is a complex tissue made of a structured combination of cell types. It has an enormous regenerative capacity and contains several different kinds of stem cells (SCs). Fundamental differences likely exist between embryonic skin stem cells and adult skin stem cells. In this chapter we focus on epithelial skin stem cells (ESSCs) in postnatal mouse skin. Nonhuman studies allow for high flexibility and increased depth of study and are less restricted by ethical issues. ESSCs are thought to reside in a specialized area of the hair follicle called ‘the bulge.’ Cells in this compartment possess the ability to differentiate, at least under stress conditions, into different cell lineages to regenerate not only the hair follicle but also the sebaceous gland (SG) and the epidermis. In adult skin, stem cells are thought to be the slow-cycling cells that retain the bromo-deoxy-uridine (BrdU) DNA label over long periods of time. How these cells maintain their properties of self-renewal and differentiation remains largely unknown. Whether these cells are fundamentally different from their progeny, or whether it is simply their location, the so-called niche, that instructs them to behave as stem cells remains an open question. Work has begun to elucidate the signaling mechanisms that control the fate of these cells in mouse skin. The challenge to isolate slowly cycling cells and to identify markers of ESSCs, which may specify unique characteristics of these cells, is now overcome, opening new and exciting avenues for future insight into understanding what maintains these cells in a dormant but potent state.