Salivary glands

The salivary glands (glandulae oris) are divided into major salivary glands (parotid, submandibular, sublingual) and minor salivary glands (glands of the oral cavity, pharynx, upper respiratory tract). The former are paired, the latter are multiple.

Among the large glands, the largest is the parotid gland, weighing 25-30 g. The submandibular salivary gland, located in the submandibular triangle of the neck, is significantly smaller in size. Even smaller is the sublingual salivary gland, which is located under the mucous membrane of the anterior part of the floor of the oral cavity.

Minor salivary glands (glandulae salivariae minores) are located in the thickness of the mucous membrane and submucosa of the oral cavity. Their size ranges from 1 to 5 mm. According to the topographic principle, the glands are divided into labial (glandulae labialea), buccal (glandulae buccales), molar (located near the molars) (glandulae molares), palatine (glandulae palatinae) and lingual (glandulae linguales) glands.

The large salivary glands are located outside the walls of the oral cavity, but open into it through excretory ducts.

Regardless of topography and size, all salivary glands (both small and large) have a common structural plan. All salivary glands have an ectolermal origin and a complex alveolar or alveolar-tubular structure. Salivary glands have a body (the main, secretory section) and an excretory duct. The body is represented by the parenchyma and stroma of the gland.

The secretory sections (initial parts) are subdivided by structure and nature of the secretion into protein (serous), mucous (mucous) and mixed (protein-mucous) sections. According to the mechanism of secretion, all salivary glands are classified as glands of the microcline type. Protein glands secrete a liquid secretion rich in enzymes. Mucous glands secrete a thicker and more viscous secretion containing a large amount of mucin - a substance that includes glycosaminoglycans.

The excretory ducts of the salivary glands are divided into intralobular, including intercalated ducts (initial parts of the ductal apparatus), and the so-called striated ducts.

Based on symptoms, various diseases of the salivary glands are distinguished, more details here.

The striated ducts of the salivary glands pass into the interlobular ducts, which give rise to the common excretory duct of the gland, which opens with an orifice on the walls of the oral cavity. The intercalated ducts are usually lined with cuboidal and prismatic epithelial cells, the striated ducts are lined with cylindrical epithelial cells, which are characterized by the presence of invaginations of the basal part of the plasma membrane. Between the invaginations there is a significant number of mitochondria, which give the cells a striated pattern. The interlobular ducts are lined with a two-layer epithelium, which gradually becomes flat. The common excretory duct of the salivary glands is usually lined with multilayered cuboidal, and in the area of the orifice - with multilayered squamous epithelium.

The excretory ducts of the various salivary glands have their own characteristics. The intercalated ducts of the submandibular gland are shorter and less branched than those of the parotid gland. The intercalated and striated ducts of the sublingual gland are almost undeveloped. By type of secretion, the lingual glands are predominantly serous. The mucous glands of the tongue are located only in the area of the root of the tongue and along its lateral sides. Mixed lingual glands are located in the anterior part of the tongue. The palatine glands are mucous, and the buccal, molar and labial glands are mixed.

The salivary glands perform an exocrine function. It consists of regular secretion of saliva into the oral cavity. Saliva contains water (approximately 99%), mucus (mucin), enzymes (amylase, maltase), inorganic substances, immunoglobulins. Saliva moistens food, moistens the oral mucosa. Salivary enzymes break down polysaccharides into disaccharides and monosaccharides (glucose).

The salivary glands consist of primary lobules (acini), which form the lobes of the gland. They are separated from each other by well-developed connective tissue, which contains various cellular elements (fat and plasma cells, lymphocytes, etc.), vessels, nerves and ducts. The lobules are represented by several blind sacs, which are the terminal, main sections. The secretory cells of the terminal sections are cubic or conical in shape and are located on a thin basal membrane. The basophilic cytoplasm of these cells contains a large number of secretory granules, the nucleus is located in the lower third of the cell. Basal (basket) cells, capable of active contraction due to the content of fibrils, are also adjacent to the basal membrane. These cells belong to the myoepithelial elements. The intercalated sections, salivary tubes, and excretory ducts, through which saliva flows sequentially from the terminal section, also contain basal cells lined with cubic or flat epithelium, salivary tubes with prismatic epithelium, excretory ducts with bilayered epithelium, intercalated sections with highly prismatic epithelium, which, as the excretory duct thickens, becomes multilayered cubic. The epithelium of the intercalated sections and salivary tubes has secretory activity.

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How do salivary glands develop?

Only vertebrates have salivary glands. Fish and whales do not have them. In some reptiles, the salivary glands are transformed into poison glands. Only mammals undergo complete evolutionary development of the salivary glands.

In the 5th week of embryonic life of the fetus, the ectodermal epithelium of the oral cavity forms a flat groove that develops into the rudiment of the parotid gland. Later, it acquires the shape of a tube, the anterior end of which contacts the epithelium of the oral cavity. The tube is surrounded by primary mesenchyme, into which the bud of the salivary rudiment grows. The rudiment of the parotid gland successively divides with the formation of acini and ducts. In the formed lumen, narrow primary excretory ducts with low cuboidal epithelium are formed. The epithelium is initially single-layered, but in a 7-9-cm fetus, the epithelial cells form two layers, and mucous secretion appears in the lumen of the duct. The epithelium of the ducts in some areas ends in alveolar-tubular outgrowths, which later form terminal sections. Goblet cells of the interlobular excretory ducts and the lining of large ducts differentiate. In the 24-week fetus, the terminal sections have two layers of cells, the basal layer is represented by myoepithelial cells. Mucous secretion of the primary acini of the duct epithelium decreases as the secretory function of the terminal sections increases. The mesenchyme that surrounds the gland is thin, loose and fibrous. In the late period of embryonic life, the gland is surrounded by a capsule. The duct, which has budded and freely penetrates the mesenchymal substance, is surrounded by blood vessels and lymphoid cells that are collected in structures similar to a lymph node. The salivary process grows into them, and as a result, a small lymph node containing salivary substance is surrounded by the parotid gland. The salivary duct and acinus are found after some time in a mature lymph node. They are observed in the deep parotid and cervical lymph nodes, located at a considerable distance from the capsule of the gland. The heterotopic structure of the salivary glands in the intraglandular and extraglandular lymph nodes explains the frequency of observed adenolymphomas in the parotid region. The salivary tubes and intercalated parts of the salivary glands develop in the postembryonic period of life.

The rudiment of the submandibular gland is of endodermal origin and appears slightly later than that of the parotid gland. Since then, it is located near the rudiment of the diffusely growing parotid gland. After some time, the endoderm of the lower segment of the oral cavity forms the rudiments of the sublingual gland. Despite the fact that the rudiments of the parotid gland appear first, the submandibular and sublingual glands are organs that have a capsule. Some salivary glands, scattered in various parts of the head and neck, are heterotopic.

Minor salivary glands are formed much later, and their rudiments appear in the epithelium of the mucous membrane of the oral cavity and pharynx (lips, tongue, hard and soft palate, tonsils, maxillary sinus, larynx, trachea). In a pathological condition, the cells of the secretory part of the duct of the SG and the epithelium are often transformed into various morphological types.

In 28% of healthy people, fat cells are found in the salivary glands. In glandular tissue adjacent to a tumor, they are found in 25% of cases. Morphologically, fat cells of the salivary glands are similar to fat cells of the skin in size, shape, and lipid content. They are usually located in the branches of the canals or in the blind ends of the interlobular ducts. The location of the fat cell reflects the specific plasticity of the duct and acinus epithelium, the ability to differentiate in many directions. Fat cells can be found in the salivary glands in a physiological state, but more often appear in inflammation and tumors. They are also found in the parenchyma of the salivary glands.

Clear cells appear in the salivary gland duct in pathological conditions and tumors. They have a cell membrane and transparent cytoplasm. The large vesicular nucleus contains a cluster of chromatin. These cells appear singly or in groups (pleomorphic adenoma) or form large fields, as in mucoepidermoid and acinar cell tumors. Histochemical studies have shown the presence of a large amount of glycogen in the cytoplasm. Glycogen-rich clear cells have the appearance of a myoepithelial cell.

Only in the epithelial cells of the acini and ducts are mitoses rare; in children, mitoses are found in the so-called "proliferation zone", but they are absent in adults. In areas with damaged glandular parenchyma, partial restoration of regeneration occurs. A hyperplastic reaction occurs in the adjacent acinus and duct. Hypertrophy and hyperplasia of epithelial components occurs especially often during inflammation. In proliferating cells, atypia and hyperplasia of the glandular and stromal elements develop, imitating tumor growth.

The parenchyma, especially of the major salivary glands, undergoes atrophy in elderly patients in the case of chronic inflammation and other pathological processes such as hypoxia due to circulatory disorders, chronic alcoholism, metabolic disorders, etc. The serous acinus of the parotid gland is most sensitive to changes leading to degeneration. This is especially true in cases of fatty atrophy, in which the glandular acinus slowly contracts and its borders become unclear. Lipid droplets appear in the cytoplasm of secretory cells, which are replaced by lipoblasts. The mature fat cell is surrounded by atrophied acini and gradually replaces them; the salivary glands degenerate. The glandular cells are adjacent to blood vessels, and the epithelium of the gland duct supports their vital activity.

Involution is the result of hyalinosis and fibrosis. Compaction and nodular formation in the gland occur, imitating tumor changes. As a result of involution, the parenchyma of the gland is compressed by proliferating fibrosis and stromal hyalinosis, and atrophies. The process usually begins with the appearance of a hyalofibrous mass in the substance surrounding the duct. Hyalinosis can accelerate as a result of inflammation and be combined with cystic degeneration of the excretory duct. The single-row epithelium of the duct flattens and slowly atrophies. The epithelium of the tubules and interlobular ducts undergoes squamous metaplasia.

Irradiation causes typical hyaline degeneration. These changes are characteristic of all salivary glands. Clinical observations confirm the development of a malignant process in irradiated areas. The first microscopic changes in irradiated tissues are gland edema and increased mucus production. Later, the salivary acinus atrophies, and the efferent duct expands cystically. The serous acinus is the most vulnerable. One of the most characteristic morphological changes after irradiation is cell atypia in the ductal epithelium and tissue fibrosis.