Fig. 1. Morphology of secretory cells in a high-pressure-frozen freeze-substituted salivary gland. (A) Part of a cross-section through a non-stimulated salivary gland. The gland is composed of a single layer of secretory cells and enclosed by a basal lamina (bl). The apical surface (a) of the secretory cells is covered by spatulate microvilli and invaginates deeply into the cell. Numerous mitochondria (m) reside close to the apical (a) and the basolateral (b) domain of the plasma membrane. lu, lumen of the salivary gland; nu, nucleus. (B) High-magnification view of spatulate microvilli on the secretory cell apical domain. The cytoplasmic aspect of the plasma membrane is studded with particles (arrowheads). In most areas of the plasma membrane, these particles are too densely packed to be revealed individually (arrows). (C) The basolateral domain of the plasma membrane lacks such particles. Scale bars, 2.5 µm (A), 200 nm (B,C).