The early changes of parietal cell structure in the course of secretory activity in the rat

The fine structure of the rat parietal cell was studied, both at rest and after stimulation by refeeding or insulin administration. Experiments on fixation procedures showed that whenever the fixative contained sucrose at a concentration higher than 0.2 M, the system of cytoplasmic membranes was clearly tubular in arrangement, whereas the omission of sucrose in the fixative usually resulted in a vesicular structure. The study with the high-voltage electron microscope of thick sections prepared by conventional techniques or by impregnation with zinc iodide-osmium (ZIO) revealed that the tubules are grouped into fascicles, and that these form a feltwork that is especially thick toward the cell apex. The development of the secretory canaliculus after stimulation appears to take place by an in situ remodeling of the cytoplasmic domain occupied by the tubular system. Cells examined after short periods of stimulation (5-15 min) showed images of the tubular system and of the canalicular structure which differed both from the nonstimulated and from the fully active (30-45 min of stimulation) cell. These features include the formation of wide cisternae and of pericanalicular cytoplasmic trabeculae or laminae, whose fine structure bears close resemblance to that of the intracanalicular processes in the same cells. These images can be ordered into a hypothetical sequence which is proposed as a model to explain the transformation of the tubular system and intervening cytoplasmic matrix into secretory canaliculus.