Читать книгу Salivary Gland Pathology - Группа авторов - Страница 45

The salivary glands are composed of large numbers of secretory acini that may be tubular or globular in shape. Each acinus drains into a duct. These microscopic ducts coalesce to form lobular ducts. Each lobule has its own duct and these then merge to form the main ducts. The individual lobes and lobules are separated by dense connective tissue that is continuous with the gland capsule. The ducts, blood vessels, lymphatics, and nerves run through and are supported by this connective tissue.

The acini are the primary secretory organs but the saliva is modified as it passes through the intercalated, striated, and excretory ducts before being discharged into the mouth and oropharynx (Figure 1.14). The lobules also contain significant amounts of adipose tissue particularly in the parotid gland. The proportion of adipose tissue relative to excretory acinar cells increases with age.

In the human parotid, the excretory acini are almost entirely serous. In the submandibular gland, again, the secretory units are mostly serous but there are additional mucous tubules and acini. In some areas, the mucinous acini have crescentic “caps” of serous cells called serous demilunes. In the sublingual gland, the acini are almost entirely mucinous although there are occasional serous acini or demilunes.

The serous cells contain numerous proteinaceous secretory (zymogen) granules. These granules contain high levels of amylase. In addition, the secretory cells produce kallikrein, lactoferrin, and lysozyme. In mucous cells, the cytoplasm is packed with large pale secretory droplets.

Initially the secretory acini drain into intercalated ducts. These ducts function mainly to conduct the saliva but they may also modify the electrolyte content and secrete immunoglobulin A. The intercalated ducts drain into striated ducts that coalesce into intralobular and extralobular collecting ducts. The intercalated duct cells are very active metabolically, and they transport potassium and bicarbonate into saliva. They reabsorb sodium and chloride ions so that the resulting saliva is hypotonic. They also secrete immunoglobulin A, lysozyme, and kallikrein. The immunoglobulin is produced by plasma cells adjacent to the striated duct cells, and it is then transported through the epithelial lining into the saliva. The main collecting ducts are simple conduits for saliva and do not modify the composition of the saliva.

Myoepithelial cells are contractile cells closely related to the secretory acini and much of the duct system. The myoepithelial cells lie between the basal lamina and the epithelial cells. Numerous cytoplasmic processes arise from them and surround the serous acini as basket cells. Those associated with the duct cells are more fusiform and are aligned along the length of the ducts. The cytoplasm of the myoepithelial cells contains actin myofilaments that contract as a result of both parasympathetic and sympathetic activity. Thus, the myoepithelial cells “squeeze” the saliva out of the secretory acini and ducts and add to the salivary secretory pressure.

Although the parotid capsule is a continuous structure covering the superficial and deep aspects of the gland, it becomes very attenuated where the gland envelopes the branches of the facial nerve. This is of some significance in parotid surgery as when peeling the gland off the branches of the nerve there may be none or a very thin capsule separating the gland from the nerve.