Sex-related changes in the population of lymphocytes in the bone marrow of the mouse. An electron microscope study.

Lymphocytes in the bone marrow were qualitatively and quantitatively examined by electron microscopy in dd-mice of both sexes at various ages. In males, marrow lymphocytes represented 4% of all the nucleated blood cells in the marrow at 4 days of age, 14% at 20 days and 21% at 35 days. From 60 days to one year of age they remained about 8%. In females, marrow lymphocytes were almost similar in proportion to those in males in early life until 35 days, and the proportion of lymphocytes, unlike that in males, remained almost unchanged thereafter until one year of age. Thus marrow lymphocytes were significantly greater in proportion in females than in males after puberty. The marrow lymphocytes were mainly small lymphocytes. The marrow small lymphocytes could be classified, by ultrastructural features, into two types: dark and light small lymphocytes. Dark small lymphocytes constituted about 90% of all the marrow small lymphocytes in early life in both sexes, but they then underwent a decrease especially in males. Light small lymphocytes were few in early life, but they increased with age in both sexes. The two types of small lymphocytes were discussed in relation to the lymphocyte subpopulations which have been generally accepted.