Gastric oxyntic cell structure as related to secretory activity

The oxyntic, or parietal cell has two characteristic membrane systems. The mammalian intracellular canaliculi are specialized networks of narrow channels lined with numerous microvilli. The other common to all oxyntic cells is the tubulovesicles, a system of tubules and vesicles. The tubulovesicular compartment is drastically depleted during maximal gastric acid secretion and this is coincident with an increase in the cell surface membrane area. A plausible explanation of this process is the fusion and transfer of tubulovesicular membranes to the plasma membrane. However, for many years there was no convincing evidence of the connections between these two membrane systems. How the tubulovesicular membranes transform into plasma membrane without demonstrable connections has been an enigma to electron microscopists. Recent ultra-high resolution scanning electron microscopic observation on the rat oxyntic cell treated with aldehyde-osmium-aldehyde method revealed that in the resting stage, the tubulovesicles were isolated spherical vesicles. But after tetragastrin stimulation, they were interconnected by slender connecting tubules forming a tubulovesicular network. Then this network was fused to the intracellular canaliculus at relatively few points. These connections between the tubulovesicles and luminal surface membrane was also demonstrated in the frog oxynticopeptic cells. In this review, these membrane transformations as well as changes of the H+/K(+)-ATPase, the lectin binding glycocalyx and the cytoskeleton during secretion will be illustrated and discussed.