Thymus organogenesis and molecular mechanisms of thymic epithelial cell differentiation.

In the mature thymus, thymocyte maturation depends on interactions with different thymic epithelial subtypes in a three-dimensional thymic architecture. However, the molecular mechanisms that generate these epithelial subtypes are not well understood. Evidence is accumulating that during fetal thymus development, epithelial cells differentiate by successive interactions with differentiating thymocytes. This review presents fetal thymus development as a process of organogenesis, the main function of which is to promote thymic epithelial cell differentiation and the generation of a functional thymic microenvironment. In this model, endoderm-derived epithelial cells are the driving force in generating the thymic primordium, with hematopoietic cells providing later signals that organize and pattern the developing thymus.