A roadmap for thymic epithelial cell development.

Thymic epithelial cells (TEC) are essential components of the thymus that guide and control the development and TCR repertoire selection of T cells. Previously, TEC have been considered as postmitotic cells that, once generated during ontogeny, were maintained in their mature state. Recently, it has become clear that TEC can be generated from common or committed medullary and cortical TEC progenitor cells in ontogeny, that stages of immature and mature TEC are phenotypically separable, and that TEC undergo a rapid turnover in a matter of a few weeks. All of these findings strongly suggest that in the adult thymus mature TEC are constantly regenerated from a pool of stem or progenitor cells, a view that renders the thymus structure potentially much more dynamic than previously thought. However, the identity of "thymus stem cells" is elusive, and developmental stages of TEC development are only beginning to be elucidated.