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Isolated live pancreatic acinar cells in physiological buffer, when imaged by using AFM (8, 19), reveal at the apical plasma membrane a group of circular "pits" measuring 0.4–1.2 µm in diameter, punctuated by smaller "depressions" within. Each depression averages 100–150 nm in diameter (Fig. 1), and typically three to four depressions are located within a pit. The basolateral membranes of acinar cells are, however, devoid of either pits or depressions. High-resolution AFM images of depressions in live cells further reveal a cone-shaped morphology (Fig. 2). The depth of each depression cone measures 15–30 nm. Similarly, both growth hormone (GH)-secreting cells of the pituitary gland and the chromaffin cell also possess pits and depression structures on their plasma membranes (7, 9), suggesting the universal presence of fusion pores in secretory cells.
Analogous to pancreatic acinar cells, examination of resting GH-secreting cells of the pituitary (7) and chromaffin cells of the adrenal medulla (9) also reveal the presence of pits and depressions on the cell plasma membrane. Depressions in resting GH cells measure 154 ± 4.5 nm (mean ± SE) in diameter. Exposure of the GH cell to a secretagogue resulted in a 40% increase (215 ± 4.6 nm; P diameter but no appreciable change in pit size.
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