A simple electrode for metallic replication

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.

Structure of the tight junctions of the human eccrine sweat gland

The human eccrine sweat gland contains two anatomically and functionally discrete segments: the secretory coil, which produces an isotonic or slightly hypertonic precursor fluid, and the coiled duct, which reabsorbs Na+ and Cl- to yield a hypotonic sweat. We examined the freeze-fracture morphology of tight junctions from isolated secretory coil and coiled duct segments to assess indirectly the contribution of paracellular ion transport in secretion and resorption in the sweat gland. In the secretory coil, tight junctions of the intercellular canaliculus and main lumen consisted of approximately 9 and 6, closely spaced, parallel or anastomosing elements, respectively. Tight junctions of the coiled duct were similar in appearance to those at the main lumen of the secretory coil. In both the secretory coil and coiled duct, and average of 2 to 3, widely spaced junctional elements were usually observed basolateral to the closely spaced junctional elements in the region corresponding to the location of the zonula adherens in Epon sections. The complexity of the tight junctions of the secretory coil exceeded what we expected for an epithelium secreting an isosmotic fluid. The elaborate tight junctions of the coiled duct support other evidence for an intermediate to high transepithelial resistance.