Occurrence of large groups of mast cells in subcutaneous connective tissue in the mouse

Skin from the mouse trunk together with panniculus adiposus and panniculus carnosus and, separately, trunk muscles, were fixed, stained with Astra blue at pH 1.0, made translucent in methyl salicylate and whole-mounted. In the connective tissue on the surface of panniculus carnosus directed towards the trunk muscles or on the surface of trunk muscles rounded and oval mast cells occurred singly or in groups from two to several dozen cells. These groups had no association with blood vessels or hair follicles. Mast cell groups were scarse in 1-month-old, clearly recognizable in 2-months-old and conspicuous in 4-months-old mice of both sexes. The number of mast cells and their number per group was larger in CFW/Ll and C3H than in Balb/c mice. Accumulation of mast cells in subcutaneous connective tissue was noted in animals from two separate breeding centers. The animals were free of ectoparasites and dermatophytes but contained some pinworms and protozoa. Elimination of these parasites, change of diet and drinking water as well as cage lining did not prevent the appearance of mast cell accumulations. These accumulations occurred in all studied mice (over 100) at the age of 2 months or older, and were also found in 1 out of 6 four-month-old hamsters and in 2 out of 6 four-month-old rats. It is suggested that mast cells accumulate in subcutaneous connective tissue in response to some undefined noxious agent. Whatever the reason of their presence, large groups of mast cells could considerably influence the results of tests performed in the skin-hypodermis area.