Freeze-fracture and lead ion tracer evidence for a paracellular fluid secretory pathway in rat parotid glands.

The morphology and permeability of tight junctions of the three major epithelial constituents of rat parotid gland-acinus, intercalated duct, and striated duct-have been examined ultrastructurally. Acinar and intercalated duct junctions (including those surrounding intercellular canaliculi) averaged two to three sealing strands, whereas striated duct junctions had five to eight sealing strands. When the permeability of the junctional complex was probed by means of a recently devised lead ion tracer technique, acinar junctions were found to be very permeable, intercalated duct junctions were somewhat permeable, and striated duct junctions were essentially impermeable to the tracer. Thus, by both morphological and tracer-permeability criteria, acinar tight junctions appear to be "leaky." These data provide strong evidence that, in rat parotid glands, a potential paracellular secretory pathway exists in the acinar region for the transepithelial passage of fluid.