A morphological and quantitative study was performed on the respiratory epithelium of human distal airways in 20 lungs removed at operation for the treatment of carcinoma. Using 1 micron araldite sections stained with Toluidine Blue for light microscopy, with further electron microscopic study where required, we have described mast cells in the epithelium of bronchioles, terminal bronchioles, and respiratory bronchioles. Examination of these cells at both microscopic levels demonstrates many of the typical features of mast cells--namely, metachromatic granules on staining with toluidine blue at light microscopic level, surface microvillous processes, and numerous dense intracytoplasmic granules with a whorled substructure at electron microscopy. Furthermore, we have identified cells devoid of granular material which may be recognised as mast cells by virtue of their nuclear and cell surface characteristics--the occurrence of a spectrum of change between those cells devoid of granular material and those with full granule content; and a few cells in which we found both typical dense, whorled granules and large lucent vacuoles devoid of such material. Quantitation of all respiratory epithelial cells by light microscopy shows that mast cells comprise 0-2% of the total population. A higher proportion of mast cells was found in the epithelium of smokers than in non-smokers. Because of the presence of many epithelial mast cells devoid of granule content, we suggest that the use of traditional 4 microns paraffin sections stained with thiazine dyes to study mast cells would lead to a marked underestimation of the epithelial population.
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