Effects of pregnancy and lactation on lymphocytes in the bone marrow of the mouse: a quantitative electron microscopic study.

Effects of pregnancy and lactation on lymphocytes in the bone marrow of the mouse were quantitatively examined by electron microscopy. Pregnancy induces a decrease in the frequency of marrow lymphocytes, and delivery causes further depletion of marrow lymphocytes. Then the frequency of lymphocytes gradually increases to normal 20 days after delivery. The frequency is restored to the normal level earlier in non-lactating than in lactating mothers. Marrow small lymphocytes, which constitute the majority of marrow lymphocytes, are distinguishable by electron microscopy into dark and light types. During pregnancy, the two types of small lymphocytes are equally decreased in proportion. Just after delivery dark small lymphocytes decrease markedly and thereafter increase gradually during lactation. Light small lymphocytes exhibit a gradual increase in proportion during lactation and reach the normal level at 20 days a postpartum. The results are discussed as compared with the effect of pregnancy and lactation on the lymphatic tissues, central and peripheral.