Epithelial Stem Cells and the Development of the Thymus, Parathyroid, and Skin

Abstract

It is evident that epithelial tissues are able to self-renew not only during normal homeostasis but also following damage. How this occurs has major implications for the development of therapies for degenerative conditions. While not fully understood, tissue self-renewal through the activation of endogenous stem cells is a likely possibility, however, regeneration through replication of pre-existing transit amplifying or mature cells may also occur. The current consensus is that stem cells reside in tissue-specific niches that are important in maintaining the stem cell state and exposing the cells to various differentiation signals. A comparative analysis of these niches from diverse tissue types has revealed similarities in signaling pathways for the maintenance, lineage determination, and differentiation of epithelial stem cells, but also has shown discrete differences. This review examines these issues, with a particular focus on the development of thymic epithelial cells. Understanding the nature of epithelial stem cell self-renewal and differentiation and the niches in which they reside will have important future clinical implications in regenerative medicine.