Histamine content and mast cell numbers in tissues of normal and athymic rats

Tissue histamine levels and mast cell numbers were determined in the skin, tongue and jejunum of female rnu/nu and rnu/+ rats aged between 5 and 29 weeks. The tongue and jejunal mucosa of rnu/nu rats had a larger mast cell density and histamine content than rnu/+. There was a marked increase in subepithelial mast cells in the skin of rnu/nu rats compared with their normal littermates, while mast cell numbers in the deep skin layer and the histamine content were similar in the two groups of rat. Subepithelial skin mast cells were smaller, of more variable shape and contained fewer granules than mast cells in the deep dermal layer, and, unlike the latter, did not emit a yellow fluorescence after treatment with o-phthalaldehyde. The results indicate that the bulk of the skin histamine is contained in mast cells residing in deep skin layers. They also support the view that the thymus may have a suppressive effect on both mucosal and connective tissue mast cells in vivo.