Microanatomy of the thymus: its relationship to T cell differentiation.

The structure of the thymus can be determined by study at the light and electron microscopic levels, but relating it to the current knowledge of the thymus's function requires an approach that combines immunological and anatomical methods. The framework of the thymus consists of epithelial cells with interconnecting processes. Lymphocytes fill the spaces between the epithelial cells. In both the mouse and human thymus, immunological staining of tissue sections demonstrates that the principal cell bearing major histocompatibility complex (MHC) antigens is the epithelial cell. Differences are noted between I-A (HLA-DR) and H-2K/D (HLA-A, B) allotypic specificities in both species. Immunoelectron microscopy confirms the epithelial nature of these cells in both species. The continued expression of thymus-type MHC antigens in the thymuses of irradiated, bone marrow-reconstituted mice strongly suggests the synthesis of these antigens by the epithelial cells. Bone marrow-derived MHC antigens are largely confined to the medulla of the thymus.