SALIVARY GLAND HISTOLOGY: STRUCTURE,SECRETORY CELLS AND DUCTS
Author: Sanketh DS, MDS
Salivary glands (SGs) are exocrine glands present in the oral cavity, whose function is to secrete saliva. Saliva is a multifunctional fluid which has antibacterial proteins, lubricates the oral mucosa, maintains integrity of teeth and helps in digestion. SGs are broadly of two types, them being major and minor SGs. Parotid, submandibular and sublingual glands form the major glands which together secrete 90% of the saliva. Minor salivary glands are small aggregates of secretory tissue and are around 600-1000 in number. They are located throughout the oral cavity except in the gingiva and the anterior part of the hard palate. Though they secrete only 10% of the saliva, minor SGs are predominantly responsible for mucous salivary secretion lubricating the oral cavity.
Salivary glands are also classified based on the type of secretory cell they possess. Serous and mucous cells are the secretory cells of the salivary gland. Hence they are also classified as serous SGs, mucous SGs and mixed SGs (SG possess serous and mucous cells). Parotid glands are serous, submandibular glands are mixed but are predominantly serous and sublingual glands are also mixed but predominantly mucous. Majority of the minor salivary glands are mucous except for von ebners glands which are serous glands (located in the troughs of circumvallate papillae and in the lateral tongue in the foliate papillae).
STRUCTURE OF SALIVARY GLAND
The salivary glands consist of secretory end pieces otherwise called acini (singular – acinus) which communicate with the oral cavity through a complex ductal arrangement. This arrangement along with the help of myoepithelial cells helps SGs secrete saliva.
Secretory end pieces house the secretory cells of the salivary glands. The secretory cells, as enumerated before, are of two types, namely serous and mucous cells. Secretory cells are also called acinar cells since they are housed in acini or secretory end pieces. The secretory end pieces are continuous with the main excretory duct of the salivary gland through intermediary ducts. The lumen of the end pieces open into the intercalated duct, which is continuous with the striated duct, which in turn opens into the excretory duct of the salivary gland. Also note that the ducts are lined by ductal cells. The secretory end pieces and intercalated ducts are associated with stellate shaped contractile cells called the myoepithelial cells. They are also called “basket cells” because they look like a basket holding a secretory unit. Myoepithelial cells are epithelial cells that also have smooth muscle like contractile properties. One among the many functions of this cell is to contract and help secrete saliva from the acini to the ducts. This architecture of the salivary glands can be compared to a bunch of grapes, where the grapes are the secretory end pieces and the stem/branches are the ducts.
Salivary glands are basically enclosed by a connective tissue capsule that braches into inter-lobar septa (singular-septum) to divide the parenchyma/functional tissues into lobes. These lobes are further divided by inter-lobular septa into lobules. A lobule houses secretory end pieces, the intercalated and the striated ducts. Hence intercalated and striated ducts are called intra-lobular ducts. Striated ducts continue to open into excretory ducts in between lobules. Excretory ducts in between lobules are inter-lobular excretory ducts. These continue and join other inter-lobular excretory ducts to form inter-lobar excretory ducts which in turn finally open into the main excretory duct of the salivary gland that is continuous with the oral cavity.
Serous cells are housed in a spherical acinus and may be 8-12 in number surrounding a central lumen. A serous cell is pyramid shaped with a basal nucleus and numerous secretory granules in the apical cytoplasm. These granules are called zymogen granules and house salivary macromolecules or proteins. The lumen of the serous acinus has finger-like extensions that continue between the serous cells. These extensions are called canaliculi. The cells are joined to each other on their lateral surfaces through junctional complexes. Each junctional complex is made of a tight junction (zona occludens), an adhering junction (zonula adherens) and a desmosome (macula adherens).
Mucous acinar cells are also pyramidal cells with the apical cytoplasm packed with mucin granules and a flat nucleus in the basal cytoplasm compressed by mucin granules. These cells are housed in tubular secretory end pieces and a cross section would show mucous acinar cells surrounding a central lumen. Sometimes, mucous acini or end pieces may be associated with serous cells arranged on the them in the shape of a crescent. This is called a serous demilune. Serous demilunes are much like serous acini. Unlike, serous acini, mucous acini lack intercellular canaliculi. However, those mucous acini associated with serous demilunes have canaliculi and the secretions of these serous demilunes reach the lumen of the end pieces through these canaliculi.
Under the light microscope, the mucin granules/ mucin cannot be seen in H & E preparations, since they get washed out. Hence the cells look empty when stained with H & E stains. However, special stains like periodic acid-Schiff and alcian blue can stain mucin/mucus.
DUCTS & DUCTAL CELLS
Secretory end pieces are continuous with the intercalated ducts. Intercalated ducts are lined by simple cuboidal epithelium and are associated with myoepithelial cells. The intercalated duct cells have a central nucleus and little cytoplasm. Intercalated ducts are very small and are usually not visible under the microscope. Several intercalated ducts join and open into striated ducts.
Striated ducts are lined by columnar cells which have a centrally placed nucleus and an eosinophilic cytoplasm. The basal end of the striated ductal cell is thrown into numerous deep folds housing several mitochondria. These deep folds give the basal surface of the cell a striated appearance under the microscope, hence the name. These ducts form the main portion of the salivary gland ductal system and are responsible for most of the ionic transport happening in the saliva when it travels from the end piece to the oral cavity. This process requires a lot of energy and this is why these cells possess several mitochondria in the basal folds of their cytoplasm.
Excretory ducts are present in the connective tissue septa between lobules and are called inter-lobular ducts. Interlobular excretory ducts fuse to form bigger inter-lobar excretory ducts, with the main excretory duct opening into the oral cavity. The inter-lobular excretory ducts are lined by pseudo-stratified columnar epithelium, and as the ducts get larger and approach the oral cavity, the lining becomes stratified squamous.
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