The Salivary glands may be classified on the basis of their secretion and the epithelium of their end-pieces as serous, mucous and mixed glands.
Of the three pairs of salivary glands in mammals, the parotid glands is the serous type, the mandibular and sublingual glands are of the mixed type.
The salivary glands are true compound tubuloalveolar glands, consisting of large and small lobules.
The glands are enveloped by a connective tissue capsule, which sends trabeculae into the interior of the organ to form interlobar, interlobular connective tissue framework.
Serous gland
- Pyramidal cells resting on a basement membrane and enclosing a narrow lumen line the serous alveoli.
- The cell boundaries are indistinct. Secretory capillaries extend between the cells and communicate with the narrow lumen of the alveolus (intercellular secretory canaliculi).
- Continuation of these from the so-called intracellular secretory canaliculi, which are infoldings of the cell membrane.
- The spherical nucleus lies in the proximal half of the cells but not against the wall.
- The serous cells show inclusions of strongly refractive secretory zymogen granules which vary in number and arrangement in the cytoplasm which is generally acidophilic.
- Between the secreting cells and the basement membrane is a layer of flat, star shaped myoepithelial cells.
- These are joined to each other by their cytoplasmic processes to form a basket like shell around the secretory cells. Therefore they are called basket cells. These are believed to assist in the discharge of secretion into the ducts by their contraction.
Ducts
- The alveoli open into narrow acinar ducts lined by cuboidal epithelium. In salivary glands, the acinar ducts are also called as intercalated ducts.
- Within the lobule, the acinar ducts unite to form Interalobular ducts, which are lined by simple columnar epithelium, with intensely eosionophilic cells, which show basal striations.
- These are believed to posses a secretory function (secretion of calcium salts) and these ducts are also referred as striated tubules or salivary ducts.
- A thin connective tissue layer, containing collagen and elastic fibres and few muscle cells, surrounds the simple columnar epithelium resting on a basement membrane.
- The interlobular ducts are also lined by simple columnar epithelium, but the cells do not show striations.
- The larger ducts show a two layered columnar epithelium and the terminal portions show stratified columnar epithelium, which changes to the stratified squamous type at the openings of the ducts.
- There is gradual increase in the connective tissue layer as the ducts become two layered. In certain instances, like parotid gland of ruminants, mast cells are found around the salivary ducts.
Mucous gland
- Pyramidal or polygonal cells resting on a basement membrane line the mucous alveoli.
- The cell boundaries are distinct and the nucleus is flattened lies against the base of the cell and stains deeply.
- Secretory capillaries are absent. With haematoxylin and eosin staining, the cytoplasm of the mucous cells stains blue.
- When full of secretion, mucous cells are often markedly distended and occupied, except for a narrow strip of basal cytoplasm by mucinogen droplets, demonstrable only by special methods.
- They may be stained with mucin stains (e.g. mucicarmine). The pressure of the contents flattens mucous cell and nucleus is pushed against the basal wall.
- At the base of the mucous alveolus, a crescent shaped groups of serous cells, referred to as the demilunes or crescents of gianuzzi. These may have direct access to the lumen of the alveolus or may communicate with it by means of intercellular secretory canaliculi, which pass between the mucous cells.
- In addition to the true demilunes there are also groups of non-secreting mucous cells called Stohr’s crescents. These lack secretory capillaries.
- The mixed glands either have separate serous alveoli or the same alveolus may contain both mucous and serous cells singly or in groups.
- In the second case, secretory unit is usually elongated and the serous cells usually occupy the terminal blind end of the alveolus, with mucous cells nearer the exit.
