Spatial relation between major histocompatibility complex‐restricted antigen receptor‐bearing thymocytes and subtypes of thymic epithelial cells

R73, a monoclonal antibody that recognizes a rat T cell surface antigen of TCR_αβ, was used to identify thymocytes that express major histocompatibility complex‐restricted antigen receptors, and to define the spatial relation between these receptor‐bearing thymocytes and individual thymic cortical and medullary epithelial subtypes by ultrastructural immunohistochemistry. We show that in both the cortex and medulla 1) the thymocytes that reacted with R73 antibodies exhibited three staining patterns: cytoplasmic‐only staining, simultaneous cytoplasmic and surface membrane staining, and surface membrane‐only staining; 2) the subcapsular/perivascular epithelial cells (ECs) were usually associated with thymocytes expressing perinuclear staining only; and 3) the surface membrane areas of thymocytes that expressed antigen receptors made contact with pale and intermediate ECs, but not with dark cells. These results suggest that thymic selection of major histocompatibility complex restriction and/or tolerance may occur by interaction of the receptors on maturing thymocytes with major histocompatibility complex antigens on thymic ECs in general, and on pale and intermediate subtypes in particular. Anat Rec 267:131–136, 2002. © 2002 Wiley‐Liss, Inc.