Ultra-high-resolution scanning electron microscopic studies on the membrane system of the parietal cells of the rat in the resting state and shortly after stimulation.

The three-dimensional organization of the membrane system of the rat parietal cells in the resting state and during early stimulation with tetragastrin (gastrin) was determined by ultra-high-resolution scanning electron microscopy. Specimens were prepared by cytoplasmic matrix removal using the aldehyde-osmium-DMSO-osmium procedure. The intracellular canaliculus was lined with numerous microvilli. Viewed from the cytoplasmic side, the intracellular canaliculi appeared as an arborized system of cactus-like structures with numerous round holes about 100 nm in diameter corresponding to the basal openings of the microvilli. The intracellular canaliculi were more developed after gastrin stimulation than in the resting state. In resting cells, most of the tubulovesicles were isolated, 100-200 nm in diameter, spherical or tubular in shape, and had a smooth surface. After gastrin stimulation, these structures were interconnected by slender tubules of about 30 nm in diameter forming together tubulovesicular network. Occasionally, swollen and shrunken profiles were observed. The tubulovesicular network was connected with the intracellular canaliculus only at a few sites by the slender connecting tubules. Fusion of the tubulovesicular network with the intracellular canaliculus is observed at such sites. In the fasted rat, the microvilli were slender and their interior was packed with some kind of ill-defined material, probably microfilaments. However, after gastrin stimulation, the microvilli were swollen and their interior was almost empty. These morphological changes seem to indicate the accumulation of fluid in the microvilli after gastrin stimulation, with subsequent swelling.