Cell surface topography of thymic microenvironments.

In the thymus gland, differentiating T lymphocytes interact, at various levels of differentiation, with the thymic stroma. This type of interaction is generally thought to be important in the "education" of T lymphocytes. This paper focuses on the complexity of the thymic stroma and identifies various types of lympho-stromal interactions, using scanning electron microscopy as a tool. We show heterogeneity at the level of stromal cells in both thymic compartments, the cortex and medulla. In addition, specialized epithelial reticulum occurs in the subcapsular area, known to be the site where immature thymocytes proliferate and differentiate. Here "basket" type epithelial structures retain groups of thymocytes. Furthermore, virtually closed lympho-stromal complexes, resembling "thymic nurse" cells are located in this area. Lymphoid cells show signs of active migration between epithelial reticular cells. These cells even transit from or into the thymic nurse cell-like structures. The medulla is characterized by other stromal elements, such as short fat-bodied epithelial cells and bone marrow derived interdigitating reticular cells. In addition, cysts lined by ciliated columnar epithelial cells occur in the corticomedullary junction area. The physiologic significance of these various microenvironments in the various steps of T cell differentiation is discussed.