The mouse thymus changes dramatically during pregnancy. It shrinks in size, and the cortex is extensively
reduced from midpregnancy onwards. Despite this, there is surprisingly little evidence for any increase in
apoptosis, and considerable evidence that mitosis of thymocytes continues throughout pregnancy. In spite of
overall involution the thymic medulla actually expands in midpregnancy due to a combination of mitosis of
epithelial cells and an accumulation of lymphocytes. The extent and nature of these changes are examined in
this study at the ultrastructural level. The epithelial cells of the subcapsular cortex (type 1 cells) become
wrinkled and exhibit powers of phagocytosis, whilst the other cortical epithelial cells are relatively
unchanged, although the formation of epithelial/thymocyte rosettes and thymic nurse cells is more clearly
seen in midpregnancy than usual. Other changes associated with pregnancy involve the medullary epithelial
cells that undergo an increased level of mitosis. Their greater numbers surround accumulations of
lymphocytes to form the characteristic medullary epithelial rings. Cell movement through blood vessel walls
was clearly observed in midpregnancy, but not at other times. Interdigitating cells in the medulla become
more conspicuous as pregnancy proceeds and the cells become phagocytic. The endoplasmic reticulum in
plasma cells becomes expanded, indicating increased secretory activity. These results highlight the active
nature of the thymus in pregnancy in spite of its involution. This picture contradicts the conventional notion
that an involuted thymus is inactive.