Morphological studies on the translocation of tubulovesicular system toward the intracellular canaliculus during stimulation of the gastric parietal cell

The gastric parietal has two characteristic membrane systems. One is the in tracellular canaliculus, which is specialized networks of enfolded luminal membrane channels lined with numerous microvilli. The other structures comm on to all parietal cells are the tubulovesicles or the tubulovesicular memb ranes, a system of tubules and vesicles. The tubulovesicular compartment is drastically depleted during maximal gastric acid secretion and this is coi ncident with an increase in the canalicular cell surface membrane. A plausi ble explanation for this redistribution is the fusion and transfer of tubul ovesicular membranes to the plasma membrane. However, for many years there was no convincing evidence of connections between these two membrane system s. The mechanism of the transformation of tubulovesicular membrane into the plasma membrane without demonstrable connections has been an enigma to ele ctron microscopists. Using a recently developed fixation technique far pari etal cells [Sugai et al. (1995) Acta Anat Nippon 74:S101], we have investig ated the organization of the cytoplasmic membrane systems in the rat restin g and tetragastrin stimulated stomachs by ultra-high-resolution scanning el ectron microscopy (SEM). Gastric mucosae were microwave-fixed in a cacodyla te buffer, (334 milliosmoles/kgH(2)O (mOsm)), to which 1.0% glutaraldehyde and 0.5% formaldehyde were added. Specimens examined by TEM of thin section s revealed the cytoplasm packed with tubular membranes similar to images de tected by rapid-freeze/freeze-substitution fixation. To render the cytoplas mic membranes visible by SEM, fixed mucosae were treated by the aldehyde-os mium-DMSO-osmium maceration procedure. With much of the cell matrix and fil aments removed, SEM revealed numerous 30-60-nm tubules, which formed a mesh work with small cisternae. Vesicles or isolated tubules were not found in a dequately macerated parietal cells. The cytoplasmic surface of the intracel lular canaliculus was smooth except for round openings representing the bas es of macerated microvilli. In favorable sites, connections of the tubular membranes to the canaliculi were clearly visible. Stereo pair views were pa rticularly useful to demonstrate these continuities. Connections between th ese two membrane compartments suggest the probability of rapid membrane tra nsposition. In this article, the form and distribution of membrane systems of parietal cells in the resting state and after tetragastrin stimulation w ill be presented and discussed. Special emphasis is made to demonstrate con nections between the tubulovesicular system and the intracellular canalicul us. (C) 2000 Wiley-Liss, Inc.