The human thymic environment.

Selected combinations of antisera labelled with different fluorochromes were used to explore the cellular interactions in the human thymic cortex and the medulla. In the early fetal thymus the cortical epithelial cells and an apparently more mobile medullary interdigitating cell population expressed large amounts of Ia-like (HLA-DR) and moderate amounts of HLA-A,B,C antigens while the lymphoid cells in the cortex expressed human T lymphocyte and cortical thymocyte antigens but were Ia-, HLA-A,B,C-. This arrangement is maintained in the infant thymus. The terminal deoxynucleotidyl transferase enzyme is first generated in cortical thymocytes (around the 17th week of gestation) and appears in bone marrow precursor cells later. In the infant thymus the lymphoid populations could be classified, according to their reactivity pattern with the new monoclonal antibodies of the OKT series, into three cell types: putative prothymocytes, typical cortical thymocytes and medullary thymocytes. Among the medullary population the majority showed the phenotype of the "inducer" cell and a minority showed that of "suppressor-cytotoxic" cells. The thymic medulla (and interdigitating cells in peripheral lymphoid organs) may predominantly contribute to the generation of "inducer" T lymphocytes.