Age- and sex-related changes of peritoneal free cells in mice: quantitative morphologic study

Peritoneal free cells in 256 mice of either sex ranging in age from 1 to 120 days were studied by quantitative morphologic procedures. The total number of peritoneal free cells examined by Coulter counter increased with age. After 40 days the increase was more marked in females than in males, so that a significant sex difference appeared in the number of peritoneal cells after puberty. The size distribution curves of peritoneal free cells obtained by a Coulter counter and channelyzer indicated that, irrespective of age and sex, peritoneal cells are composed of three populations which are each different in cell size. By light microscopy, the three components of peritoneal cells corresponded to small lymphocytes(type I cells), medium-sized mononuclear cells (type II cells), and macrophages (type II cells). In newborn mice, type III cells were about 80% of peritoneal free cells, and type I and II cells were less than 2%. Type I cells increased slowly in number with age, and they remained smallest in number at any age. Type II cells increased rapidly with age, and they were most numerous after puberty. Moreover, type II cells were significantly more numerous in females than in males. The sex difference of type II cells accounts for that in the total number of peritoneal cells. Type III cells also increased in number wtih age until puberty. After puberty they remained almost unchanged in number. The sex difference in the total number of peritoneal free cells and the number of type II cells was considered to be caused primarily by the testis but not by the ovary.