Ultra-high-resolution scanning electron microscopy of the continuity of cytoplasmic and luminal membranes in frog oxyntic cells.

Despite numerous previous studies, the presumed continuity of the luminal and tubulovesicular membranes in frog oxyntic (oxyntico-peptic) cells remains to be convincingly demonstrated. This study was undertaken to clarify this question by ultra-high-resolution scanning electron microscopy of specially prepared frog stomach specimens before and during histamine stimulation. Fasted Japanese meadow frogs stimulated with histamine were used. Oxyntic cell cytoplasmic matrix was removed by the aldehyde-osmium-DMSO-osmium maceration procedure and impregnated with osmium-hydrazine. Specimens were examined without metal coating. In the resting oxyntic cell, the luminal membrane had closely packed surface folds forming a rather flat surface with a few short microvilli. In the cytoplasm, flattened 200-500 nm vesicles were interconnected by numerous slender 20-60 nm tubules forming the tubulovesicular network. Occasional slender tubular branches were found in continuity with the luminal membrane. After histamine stimulation, the number and length of microvilli and surface folds increased, whereas the tubulovesicular membrane system decreased. Sites of clear continuity between the luminal and tubulovesicular membranes were not abundant but were clearly demonstrated in histamine-stimulated oxyntic cells. The small size of the tubules connecting the tubulovesicular system to the plasma membrane renders this observation by transmission electron microscopy very difficult. In these specimens, the clear continuity of the tubulovesicular network to the luminal plasma membrane became more evident. This study confirms previous findings of increased luminal membrane and depletion of the tubulovesicle system. The demonstration of continuity between these two compartments in our SEM preparations supports the hypothesis of direct transfer of tubulovesicular membrane to oxyntic cell luminal secretory membrane.