Diagram comparing the components of the salivon in the three major salivary glands. The four major parts of the salivon, the acinus, intercalated duct, striated duct, and excretory duct, are color coded. The three columns on the right of the salivon compare the length of the different ducts in the three salivary glands. The red-colored cells of the acinus represent serous-secreting cells, and the yellow-colored cells represent mucous-secreting cells. The ratio of serous-secreting cells to mucous-secreting cells is depicted in the acini of the various glands.
Serous demilunes are artifacts of the traditional fixation method
As noted above, each mixed acinus, such as those found in the sublingual and submandibular glands, contains serous and mucus-producing cells. In routine preparation for both light and electron microscopy, serous cells have traditionally been regarded as the structures that make up the demilune. Recent electron microscopic studies now challenge this classical interpretation of the demilune. Rapid freezing of the tissue in liquid nitrogen, followed by rapid freeze substitution with osmium tetroxide in cold acetone, reveals that both mucous and serous cells are aligned in the same row to surround the lumen of the secretory acinus. No serous demilune is found. Sections prepared from the same specimen by conventional methods show swollen mucous cells with enlarged secretory granules. The serous cells form typical demilunes and are positioned in the peripheral region of the acinus with slender cytoplasmic processes interposed between the mucous cells. These findings indicate that the demilune observed in light or electron microscopy is an artifact of the routine fixation method (Fig. 15.23). The process of demilune formation can be explained by the expansion of mucinogen, a major component of secretory granules, during routine fixation. This expansion increases the volume of the mucous cells and displaces the serous cells from their original position, thus creating the demilune effect. A similar phenomenon is sometimes seen in the intestinal mucosa, in which swollen goblet cells displace adjacent absorptive cells.
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