Thymus dependence of connective tissue mast cells: a quantitative cytofluorometric study of the growth of peritoneal mast cells in normal and athymic rats.

The normal growth of peritoneal mast cells was studied in athymic and heterozygote rats over the period of 5-29 weeks of age. The total peritoneal mast cell mass and the mass of the granular components was calculated from mast cell numbers and their content of protein, heparin and 5-hydroxytryptamine. The growth process was analyzed by allometric log-log plots of mast cell quantities versus body weight and linear regressions. The mast cell growth in both groups of rats conformed to the allometric principle and was strictly proportional to the growth of the body as a whole. Two major differences between athymic and heterozygote rats were observed. The total peritoneal mast cell mass and the mass of its components was initially higher in the athymic rats, but the growth rate of the mast cells was lower. We suggest that the thymus may regulate the mast cells by an inhibitory factor acting on the bone marrow stem cell or circulating precursor level. The lower growth rate of the athymic rats may be due to the absence of a second, stimulatory thymic factor acting on the tissue precursor level, or to a tissue homeostatic mechanism triggered by the large initial mast cell mass and unrelated to the thymus.