Major Salivary Glands
The major salivary glands are classified as exocrine glands and include three paired glands: the parotid, submandibular and sublingual glands. These glands deliver their saliva, a thin watery suspension of enzymes, mucus, inorganic ions and antibodies, into the oral cavity. Unlike the minor salivary glands, a capsule of dense connective tissue surrounds the major salivary glands. The capsule sends septa into the gland substance, subdividing the secretory acini into lobes and lobules. In addition, the septa house the large blood vessels and excretory ducts. Both the major and minor salivary glands contain numerous lymphocyte and plasma cells surrounding the secretory acini.
The salivon is the basic secretory unit of the salivary glands and consists of the acinus, intercalated duct and excretory duct. The acinus, the secretory unit of the salivary glands, is a blind sac composed of secretory cells. The salivary glands contain three types of acini: serous acini, mucous acini and mixed acini.
Serous Cells
The serous cell is a pyramidal-shaped cell and has a wide basal surface facing the basal lamina and a small apical surface facing the lumen of the acinus. The basal cytoplasm of the serous cell contains a large amount of rER, free ribosomes, a prominent Golgi apparatus and numerous spherical secretory granules. However, the apical cytoplasm of the serous cell contains its secretion, which is stored in zymogen granules. As a result, in H&E staining, the basal cytoplasm of the serous cell is stained with haematoxylin, whereas the apical cytoplasm is stained with eosin. In the TEM, the serous cells are joined to each other near their apical surface by the junctional complex. Mucous Cells
The mucous cell is a round-shaped cell. Its mucus secretion is stored in mucinogen granules in the apical cytoplasm. After these granules are discharged, it is difficult to distinguish the mucous cell from an inactive serous cell. In H&E staining, the apical portion of the cell usually appears empty because the mucinogen is lost. TEM preparation shows that the basal cytoplasm of the mucous cell contains rER, mitochondria, other components and a flattened nucleus against the base of the cell. Rapid-freeze preparation shows round mucous cells containing round nuclei in their centres. The apical cytoplasm contains numerous mucinogen granules and a large Golgi apparatus. Salivary Ducts
There are three salivary ducts in the major salivary glands: intercalated duct, striated duct and excretory duct. Intercalated Duct The intercalated duct receives secretion directly from the lumen of the salivary acinus and empties into a larger duct. In serous-secreting salivary glands, the intercalated ducts are prominent and well developed, and modify the serous secretion by both absorbing chloride ion and secreting bicarbonate ion to form the final product. However, in mucous-secreting salivary glands, in which the secretion is not modified, the intercalated ducts are short and very poorly developed. Striated Duct The striated duct is composed of a simple cuboidal epithelium. As the striated duct approaches the excretory duct, the epithelium gradually becomes simply columnar, presenting infoldings (striations) at the basal plasma membrane. These infoldings contain elongated mitochondria, which are associated with the reabsorption of fluid and electrolytes. As with intercalated ducts, striated ducts are prominent in serous glands and are short and difficult to identify in mucous glands. The striated duct modifies the secretion through the reabsorption of Na+ from the primary secretion, and the secretion of K+ and HCO3- into the secretion. The secretion becomes isotonic when it contains more Na+ and less K+, and it becomes hypotonic when it contains more K+ and less Na+. The striated duct has a larger diameter than the secretory acinus. In the parenchyma of the glands, striated ducts are found as intralobular ducts and may be surrounded by small amounts of connective tissue containing small blood vessels and nerves. Excretory Ducts Excretory ducts are the principal ducts of the major salivary glands and empty into the oral cavity. Small excretory ducts are lined with a simple cuboidal epithelium. As excretory ducts approach the oral cavity, the diameter of the duct increases and the epithelium gradually changes into a stratified columnar epithelium. Saliva
The major digestive glands produce about one litre of saliva per day, approximately 95% of the daily salivary secretion. The secretory component of these glands forms the primary saliva, which is modified by the initial portion of the duct system (striated ducts) to form the secondary saliva. The saliva’s components include water, proteins and glycoproteins (enzymes and antibodies) and electrolytes. The saliva’s functions include the following.
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Major Salivary Glands |
Histology Text: Parotid Gland - Sublingual Gland - Submandibular Gland - Liver - Gallbladder - Pancreas
Histology Atlas - Histopathology - Histology Quiz
Histology Atlas - Histopathology - Histology Quiz