| The Salivary Gland There are three major paired salivary glands: the submandibular, the sublingual and the parotid glands. They differ from one another in the relative abundance of serous and mucous acini, and in the length of the various kinds of ducts. The minor salivary glands are located in the submucosa of different parts of the oral cavity. We will look only at the histology of the submandibular gland. The secretions of the major and minor salivary glands is called saliva. On the basis of the weight of the glands producing it, the volume of saliva exceeds that of other digestive organs by as much as 40 times. Saliva moistens the oral mucosa as well as dry food before swallowing. Its high bicarbonate content buffers the oral cavity. It provides a medium for food materials to stimulate taste buds. It begins the digestion of carbohydrates via the digestive enzyme amylase. It controls the bacterial flora by secreting lysozyme. In the absence of saliva, infections and caries would develop in the oral tissues. The salivary glands also secrete IgA and potassium, and resorb sodium. The submandibular gland is a compound tubuloalveolar gland. It is surrounded by a capsule of moderately dense connective tissue, from which septa divide the gland into lobes and lobules. The secretory endpieces consist of a more or less spherical mass of cells called an acinus (pl. acini). [Some people use the term alveoli for acini. However, the term alveoli is also used for the air sacs in the lung, which are not related to the secretory endpieces of the salivary glands.] The cells of the acini appear triangular in sections, with their apex directed toward the lumen, and their base resting on a basement membrane. They secrete their product in a merocrine fashion into the lumen. Contractile cells called myoepithelial cells or basket cells lie between the basement membrane and the plasma membrane of the secretory cells. They are also found in the proximal part of the duct system. Myoepithelial cells posess many actin-containing microfilaments, which squeeze on the secretory cells and move their products toward the excretory ducts. They can sometimes be identified in standard histological preparations. Acini can be either serous or mucous. The secretion of serous cells is thin, watery and proteinaceous. Serous cells have a rounded nucleus and secretory granules in their cytoplasm. They are joined near their apical surfaces by junctional complexes. Mucous cells secrete a viscous, glycoprotein-rich product, which is stored as mucinogen granules. The nuclei are typically flattened against the base of the cells (unless the cells have just discharged their contents, in which case they look more like serous cells). Mucous cells typically look pale and empty in standard histological sections, because their granules are lost during preparation. The submandibular gland of humans is predominantly serous. Its mucous acini are quite frequently capped with a serous demilune, a crescent of serous cells around one or more of their surfaces. Because of small size of the lumen of acini and the variability in sectioning, lunina are rarely seen in the acini. There are three types of ducts in the submandibular gland: intercalated, secretory (aka striated) and excretory ducts. Intercalated ducts are slender ducts continuous with the terminal acini, and lined with flat, spindle-shaped cells. They secrete bicarbonate ion into and absorb chloride ion from the acinar product. Secretory ducts have eosinophilic cuboidal to columnar cells with basal striations. These result from infoldings of the basal membranes in which are found many mitochondria. Secretory ducts resorb sodium and secrete potassium. As they approach the excretory ducts, their diameter may exceed that of the acini. Both intercalated and secretory ducts are found within the parenchyma of the gland and are therefore intralobular ducts. The largest ducts are the excretory ducts. They are found in the connective tissue septa, and are therefore interlobular ducts. They ultimately connect with the oral cavity. Their epithelium is variable, it can be simple cuboidal, stratified cuboidal, stratified columnar or pseudostratified. Near the oral cavity, it becomes stratified squamous. Excretory ducts do not change the secretory product.
Several mucous acini are also in the field of view, identifiable by their paler staining and more flattened nuclei. Mucous acini tend to be more elongate and more irregularly shaped than serous ones. In two of the ones labelled, the lunina can easily be identified. The lumen can also be seen on the one on the right, but you have to look more closely. Serous demilunes can be seen on two of the labelled mucous acini. The one on the left has large demilunes at either end, the one on the right has a very large cluster of serous cells on its right side. The secretory duct in the centre of the field is very large, larger than the surrounding acini. Upon close inspections, striations can be seen at the base of some of its cells. (Striations are easier to see under the microscope than on these images.) Secretory ducts increase in size as they approach the excretory ducts and sections are highly variable. They can be larger or much smaller than the one seen here.
The serous acini are quite irregular in shape but many are more or less spherical. A lone fat cell is seen.
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