Intercellular junctions in upper airway submucosal glands of the rat: a tracer and freeze fracture study

The structure of intercellular tight junctions of rat airway submucosal glands was examined by freeze fracture techniques and their permeability assessed by the use of colloidal lanthanum. The submucosal glands were organized into three distinct regions: a) serous tubules and b) mucous tubules lined, respectively, by serous and mucous cells, and c) ducts lined by cuboidal epithelial cells, containing few secretory granules, and some ciliated cells. The mean number of parallel fibrils constituting the tight junctions between serous cells was 3.6 +/- 0.4, which was significantly smaller than those between any of the other cell types. Colloidal lanthanum permeated the tight junctions between serous cells up to the level of the acinar lumen. There was a progressive increase in the mean number of parallel fibrils of tight junctions between mucous (5.1 +/- 0.6), ductal (5.4 +/- 0.5), and ciliated cells (8.5 +/- 0.7); none of these junctions was permeated by colloidal lanthanum. These results imply that tight junctions between serous cells are more permeable to small water-soluble solutes than those present in the more proximal portions of the gland. Gap junctions were observed between serous cells and between mucous cells, suggesting that these secretory cells may be electotronically and metabolically coupled.