Electron-Microscopic Detection of Cytoskeletal Actin in the Epithelial Cells of Bovine Abomasum

The fine structural distribution of cytoskeletal actin was investigated in the oxyntic gland region and in the pyloric gland region of bovine abomasal mucosa using immunoelectron methods. In all exocrine cell types of the gastric epithelium, actin filaments exhibit a polarized pattern of distribution. In the members of the mucus- and protein-synthesizing cell line, the actin filaments form a prominent barrier to the exocytosis of secretory granules underneath the apical membrane. Therefore, actin is supposed to be involved in the control of exocytosis. The basolateral subplasmalemmal cortex of the exocrine cells, however, shows weaker labeling for actin filaments. In this position actin might be responsible for the movement of epithelial cells along the length of the gastric gland during physiological cell renewal and for the maintenance of the integrity of the epithelial unit and of cell polarization. In parietal cells of bovine abomasum, actin filaments are, in addition, associated with the intracellular canaliculus, but not with the tubulovesicular compartment. We assume that actin filaments reorganize the canalicular membrane after membrane translocations in the course of acid secretion. For the first time, actin filaments were documented in the rare brush cell type of bovine abomasum, where they do not form a terminal web beneath the apical membrane and therefore point out the nonexocrine function of the brush cell type. In all polarized cells of the bovine abomasum, actin filaments underlay the two apical members of the junctional complex, the tight junction and the adherens junction, where they might be involved in the regulation of cell adhesion, cell motility and cell shape determination.