Developmental stages in the human thymus.

The thymus is populated by hematopoietic cells that have the capacity to develop into at least three different hematopoietic lineages, T, NK and dendritic cells. While developing into T cells these cells pass a series of developmental stages that can be discriminated on the basis of expression of a number of antigens. The availability of a myriad of monoclonal anti- bodies against human differentiation antigens has permitted a detailed analysis of the various cellular stages in the human thymus. This analysis not only comprised investigation of molecular but also of functional features of purified thymocyte subsets, since more recently assays were set up that allowed investigation of the hematopoietic precursor activities of human thymic progenitor cells. Here we review the current status of knowledge with regard to early developmental stages in the human thymus. In addition, we discuss recent data on later developmental stages, in particular concerning positive selection and maturation of T cells.