- Normal T cell development depends upon an interaction between progenitor cells and their microenvironment. Previously raised monoclonal antibodies to subtypes of human thymic epithelium were used with gold-coupled reagents in immuno-electron microscopy to study the fine cellular distribution of the molecules to which these Mabs bind. Mab MR6, thought to recognize the human IL-4 receptor, shows strong surface labeling of cortical epithelial cells and thymic nurse cells, and very weak staining of thymocytes, medullary macrophages and interdigitating cells. Mab 1st 8.18 labels the surface of Hassall's corpuscles and associated medullary epithelial cells. The molecules detected by these two antibodies are therefore located in a position where they are available to interact with external cellular and soluble signals within the thymus. In contrast Mabs MR10 and 19 recognize Intracellular molecules within subcapsular, perivascular and some medullary epithelium. These molecules may represent soluble material awaiting secretion from the cell; alternatively, they may be internal structural proteins.