The glycoproteins of secretory cells in airway epithelium.
Quantitative histochemical methods demonstrate a variety of glycoproteins within mucus-secreting cells of airway epithelium. A single cell may synthesize one or a combination of four major types--(i) neutral glycoprotein, (ii) and (iii) sialylated either sensitive or resistant to sialidase and (iv) sulphated. In human airway disease, or in experimental response to inhalation of an irritant, there is mucus cell hyperplasia and change in the proportion of cells synthesizing the various types. Experimental studies show how speedily these changes occur. In rats exposed to tobacco smoke changes are found within 20 h of the first exposure. Only in the extrapulmonary epithelium is there discharge of the secretions, with an apparent fall in cell number. Modification of glycoprotein may occur with an unchanged or increased cell number, suggesting that it occurs in existing and newly appearing secretory cells. Modification of the contents of the granule occurs toward the cell apex. Modification of glycoprotein synthesis towards the normal is also the most sensitive and earliest sign of recovery.