The Pancreas and Salivary Glands

Study the parotid gland (slide #38). Under very low magnification note the separation of the gland into lobules by septa of connective tissue containing large, pink interlobular ducts as well as blood vessels and nerves. Often the septa appear to be embedded within the lobules as the architecture is not completely regular. With the 10x lens, note that this gland is composed predominantly of dark-staining serous acini and ducts. The latter are the acidophilic (pink) structures with round nuclei in the center of tall cells. Examine serous acini and note the rounded basally located nuclei and dark granular staining of the cytoplasm. Usually the gland lumen is not easily seen. The acini lie in a fine network of connective tissue. The absence or scarcity of mucous acini and the presence of adipose tissue between acini, i.e., in intralobular locations, aid in the identification of the parotid gland. The lumen of each individual acinus opens into an intercalated duct (first portion of the intralobular duct) which is lined with low cuboidal, pale-staining cells. In cross-section they are smaller than secretory acini, are more acidophilic, and tend to have more centrally located nuclei. Try to find longitudinal sections showing the continuity between secretory acini and intercalated ducts. More often, the continuity between intercalated ducts and the next portion of the duct system, the striated ducts, can be seen. Striated ducts are lined by cuboidal to columnar acidophilic cells and are so-named because of characteristic striations in the basal portions of the cells which are due to the alignment of mitochondria between deep infoldings of the basal cell membrane. The nucleus is in the central part of the cell above the striations. The latter are sometimes difficult to discern because of postmortem changes.

The striated ducts empty into interlobular ducts in the connective tissue septa where they become larger in diameter. Interlobular ducts fuse to form progressively larger structures. Those with walls consisting of two layers of cells are called excretory ducts. The cells bordering the lumen in these ducts usually have rounded apical surfaces.

Study the submandibular gland (slide #39). The duct system is similar to that of the parotid. Note that this gland is composed mostly of dark-staining (purple) serous acini, but also contains a substantial number of pale (pink) staining mucous acini. In general, the nuclei of mucous cells are flattened and lie close to the basement membrane. Contrast those with the rounded, more central nuclei of serous acini. Groups of dark-staining serous cells form caps called "serous demilunes" often seen in sections as crescents which partially surround some of the mucous acini and empty their secretion into a common lumen. Other features are as described for the parotid gland.

The sublingual gland (slide 48), even, is a mixed gland but predominantly mucous. It has very short striated ducts which are extremely difficult to find and lacks intercalated ducts. The intralobular ducts present are called "mucous tubules" and cannot be distinguished from the secretory end-pieces in conventional sections. Excretory ducts of varying sizes may be seen as well as some skeletal muscle.

Examine the human pancreas (slide #41) at low magnification and note its lobular nature. Some regions may be poorly preserved. The connective tissue capsule may or may not be present but note the thin septa penetrating into the gland, dividing it first into lobes and then lobules. Distinguish between basophilic exocrine and the slightly acidophilic endocrine tissue. The Islets of Langerhans vary in size and usually appear as round cords of light pink cells. At 40x magnification note the different appearance of islets and exocrine cells. The islets make up only one percent of the total pancreatic mass and are not uniformly distributed within the organ. It is possible to have sections lacking islets. Try to find a more or less round cluster of exocrine cells forming an acinus. Note the details of the secretory cells such as the dense basal basophilia and the brightly eosinophilic secretory granules in the apical half of the cells. A capillary network surrounds the acini but is difficult to distinguish.

Find the intralobular ducts among the closely packed acini. The pancreas lacks striated ducts and the intralobular duct is similar to the intercalated duct of salivary glands. It often has a collapsed lumens and the cells are cuboidal with little cytoplasmic staining in contrast to acinar cells. They are responsible for secretion of a bicarbonate-rich fluid. Locate centroacinar cells of the exocrine pancreas. These are duct cells which form the junction between the secretory endpiece and the duct. The ease of identification will depend upon the staining, so study both slides #40 and #41. Find well-defined acini in a well preserved region of the slide. The nuclei of centroacinar cells appear to be located in the center of the acini and mark the beginning of the intralobular ducts. Occasionally longitudinal sections of the first, narrow, free portions of the intralobular ducts can be observed emerging from the acini. Finally, the interlobular ducts can be seen in the connective tissue septa along with some blood vessels.