Quantitative observation of megakaryocytes in the spleen and bone marrow of the mouse: effects of sex, sex hormones, pregnancy and lactation.

Megakaryocytes in the spleen and bone marrow of the mouse were quantitatively examined particularly in relation to sex. In the splenic red pulp, megakaryocytes increase equally in number in both sexes in early life until 35 days of age. At 70 and 150 days of age, however, the number of megakaryocytes is significantly greater in females than in males, and thus a significant sex difference is apparent between the sexes. Removal of the testis causes an increase in the number of splenic megakaryocytes. In males, gonadectomized or normal, estrogen induces a marked increase in the number of megakaryocytes, whereas neither testosterone nor progesterone causes significant changes. In females, splenic megakaryocytes show a marked increase during pregnancy. After delivery, they gradually decrease in number. The decrease is more rapid in non-lactating than in lactating mice. In the bone marrow, no significant sex difference is evident in the number of megakaryocytes per unit area. Estrogen causes a significant increase in the megakaryocyte count also in the bone marrow. Based on the results obtained, megakaryocytopoiesis in the hemopoietic tissue was considered from a standpoint of its relation to sex.