Histological changes in the thymus during mouse pregnancy.

We studied the cells and microenvironments of the thymus in the pregnant mouse. A morphological and histological study at the light and electron microscope levels shows that the thymus continues to remain active despite massive loss of cortical cells. Many changes are seen in the cortex, but the medulla is relatively unaltered. In very early pregnancy a reduction in cell death and an increase in the numbers of blast cells and mitotic figures indicate a greater activity than in virgin animals. Later in pregnancy, when two-thirds of the cortical cells are lost, there is a continuing presence of mitotic figures and blast cells. Death of thymocytes by apoptosis occurs in associations (rosettes) with macrophages or cortical epithelial cells in the subcapsular and outer cortex. Death of thymocytes with adjacent epithelial cells also occurs in the deep cortex, as it does in virgin females. These results suggest that involution in pregnancy is structurally different from that induced by acute hydrocortisone treatment or other drugs. This supports the view that the term thymic involution describes not one distinct process, but several different processes. The relevance of these findings to the deletion of thymocytes reactive to paternal and fetal antigen is discussed.