Examination of the surface topography of canine gallbladder epithelium using the scanning electron microscope reveals subtle changes in the morphology of the cellular components between the full and empty states. In the full state large bays exist between relatively low, ridge-like crests or folds of mucosa which gradually increase in height as the gallbladder empties causing a subsequent reduction in size and volume of the bays. The relaxed gallbladder exhibits an irregular arrangement of tall, closely apposed mucosal folds with intervening deep valleys. The degree of apical convexity of individual epithelial cells will increase as the mucosal folds begin to relax. Other mucosal features most easily observed in the full state were small pit-like intraepithelial glands. The apical surfaces of the epithelial cells exhibit a densely uniform microvillus border and occasional membrane-bounded cytoplasmic protrusions (apical bullae) extending out into the lumen of the organ. The lateral cell membranes and basal lamina were also examined. Correlative methacrylate and transmission electron microscopic findings are included. The functional aspects of these morphological alterations and apical protrusions are discussed.
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