Ultrastructural and histochemical observations of respiratory epithelium and gland

Secretory products of epithelial cells of the human respiratory tract have been studied biochemically and by a variety of histochemical methods for differentiating and characterizing complex carbohydrates at the light and electron microscopic level. By light microscopy a majority of mucous secretory cells of the surface epithelium secret glycoprotein with terminal sialic acid, penultimate galactose residues, and variable sulfate esters. Ultrastructurally the mucous cells of the surface epithelium vary within and between regions of the respiratory tree and comprise a heterogeneous population of cells differing in the fine structure and cytochemistry of their secretory granules. Serous tubules and demilunes in glands of the lamina propria produce a secretion that contains less carbohydrate than that in the nearby mucous cells, resembles the latter in content of sulfate esters, and differs in having little or no sialic acid and no terminal or penultimate galactose. Mucous tubules produce a carbohydrate-rich secretion containing glycoconjugate with terminal sialic acids, penultimate galactose residues, and a variable degree of sulfation like the glycoconjugate of surface mucous cells. Heterogeneity of cells can be demonstrated in the serous and mucous tubules by ultrastructural, morphologic, and cytochemical methods.