Substructure of granules from serous cells of porcine tracheal submucosal glands.

The ultrastructure of serous cells from porcine tracheal submucosal glands was studied by conventional transmission electron microscopy (TEM), and by cytochemical methods to stain for complex carbohydrates. In tissue fixed and processed for TEM, and stained with uranyl acetate and lead citrate, the condensing granules of serous cells occasionally possessed a hexagonal and sometimes a lamellar substructure. Tissue fixed in paraformaldehyde-glutaraldehyde and stained with periodic acid-thiocarbohydrazide-silver proteinate (PTS) or with phosphotungstic acid (PTA) showed secretory granules stained for complex carbohydrates and revealed a substructure similar to that noted in the condensing granules. The dark staining substructure revealed by either the PTS or the PTA technique appeared to correspond to electron-lucent areas observed in the condensing granules by conventional TEM. The PTS staining probably demonstrated the presence of neutral glycoprotein, since the serous-cell granules did not react with a dialyzed iron stain for acidic glycoproteins. Treatment of periodic acid oxidized thin sections with pronase or pepsin prior to thiocarbohydrazide and silver proteinate treatment decreased the intensity of the PTS staining, but did not digest away any components of the granules. The substructure revealed by the carbohydrate stains may be a reflection of the mechanism of packaging or the macromolecular structure of the glycoproteins in the serous-cell granules.