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.
Figure 1 shows a low power view of the submandibular gland. At this magnification, the paler-staining mucous acini stand out among the predominantly serous acini. Some secretory ducts can also be identified by their circular profile and larger lumen. Adipose (fat) cells are scattered throughout the gland. A septum runs through the field of view, and the gland has separated a bit in this area during preparation. Only a few strands of yellowish connective tissue can be seen along the delicate septum. The lobule below the septum is almost entirely serous (at least the part in the field of view), in the upper lobule quite a large number of mucous acini are also seen. Some of the septa you will see as you scan your slides will be much larger than this one.
Figure 2 shows a high power view of some of the submandibular gland parenchyma. Much of the field of view is taken up by two large blood vessels and a secretory duct. As is typical, most of the acini are serous. The two serous acini that have been labelled have a round profile and the outlines of some of their triangular cells can be seen. These two sections appear rather like a pie, with the cells resembling the cut pieces of the pie. Nevertheless, the lumina are not easily identifiable. Sections of serous acini are usually more irregularly shaped, as evidenced by most of the surrounding serous acini.
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.
Figure 3 shows an intercalated duct cut in longitudinal section and surrounded by serous acini. Notice how the duct essentially blends into the surrounding acini with no marked difference in staining. It is recognized by its more flattened cells and larger lumen. Above the clearly identifiable longitudinal section of the duct (labelled), it is cut tangentially and appears as a blur of cells before emerging, fairly indistinctly, in cross section (labelled) at the top of the field of view.
The serous acini are quite irregular in shape but many are more or less spherical. A lone fat cell is seen.
Figure 4 shows a high power view of a section containing many mucous acini. They are more elongate and irregular in shape than serous acini, and may contain numerous outpockets. This is well illustrated by the large acinus to the centre left, which also has several demilunes. The lumina of many of the mucous acini can be identified. Aside from the demilunes, a few serous acini can also be seen.
Figure 5 shows a low power view of the submandibular gland. Parts of two lobes and the connective tissue surrounding them can be seen. At this magnification, no details of the acini can be distinguished (they appear as solid pink profiles), nor can intercalated ducts be identified. Secretory ducts can be seen lying among the acini; they can be identified by their darker staining and obvious lumina. Secretory ducts empty into the excretory ducts which lie in the connective tissue (hence they are interlobular). The pink staining material seen in some of the ducts is the secretory product of the acini. At this magnification, it is not possible to distinguish all of the profiles in the connective tissue septa, some may be blood vessels. Two blood vessels are clearly identifiable by the fact that they are filled with red blood cells.
Figure 6 shows a high power view of part of the wall of an excretory duct. The area shown has stratified columnar epithelium, with a row of columnar cells sitting on a row of cuboidal cells. Secretory material is seen in the lumen of the duct. Nerve bundles and blood vessels can also be seen in the connective tissue. They are sometimes much larger than those seen here.
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