Changes in the murine thymus during pregnancy were studied using immunocytochemistry with monoclonal antibodies against thymic epithelial, neuroendocrine, and thymulin-producing cells, fibroblasts, blood vessels and connective tissue components. Extensive alterations occur in mid-pregnancy. The medulla was greatly enlarged in the involuted thymus, and there were greater numbers of epithelial cells. These epithelial cells had an altered distribution forming large structures surrounding spherical masses of mononulear cells, lacked epithelial cells and often contained a central blood vessel with fibroblasts and connective tissue. We have called these structures ‘medullary epithelial rings’ (MERs). To our knowledge these structures have not been described before. Late in pregnancy the loss of the central mononuclear cells leaves collapsed structures in a smaller medulla that nevertheless retains many epithelial cells. In virgins and early-pregnancy, there are cortical channels free of epithelial cells that are very infrequent later in pregnancy. This may reflect the loss of steroid-sensitive thymocytes from the cortex. The influence of sex-steroids neurological impulses and immune activity in causing the changes are discussed, as are the possible consequences in pregnancy of a reduced, thymocyte-depleted cortex and an enlarged medulla that shows great complexity and activ
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