General Structural Plan
From a histological standpoint, all major salivary glands are classified as branched compound. This means that both the duct system branches ("compound") and the secretory units themselves branch ("branched").
Each gland consists of two primary components:
- Stroma (supporting framework). The organ is externally covered by a dense connective tissue capsule. Septa extend inward from the capsule, dividing the gland into lobes and lobules. Blood vessels and interlobular ducts course through these connective tissue layers.
- Parenchyma. Includes secretory (terminal) units, where saliva is produced, and a system of ducts that transport saliva into the oral cavity.
Terminal Units (Acini) and Their Cells
Depending on the secreted product, terminal units are divided into three types. All of them contain myoepithelial (basket) cells — flattened, stellate structures of epithelial origin. They lie between the basement membrane and the secretory cells; upon contraction of their processes, they squeeze the secretion into the ducts.
- Serous (protein) units. These have an alveolar (rounded pouch) shape. They are formed by serocytes — small pyramidal cells with a central round nucleus. Due to active protein synthesis, their cytoplasm is strongly basophilic (appearing dark under a microscope).
- Mucous units. These have a tubular shape. They consist exclusively of mucocytes. These are large cells with a pale, "foamy" cytoplasm (accumulating mucus) and a flattened nucleus displaced toward the basal pole.
- Mixed (protein-mucous) units. These are widened tubules. The central portion is formed by pale mucous cells, while the periphery features a second layer of protein cells. This proteinaceous "cap" is called a serous demilune (demilune of Giannuzzi). The protein secretion from the demilune flows into the lumen of the unit via special intercellular canaliculi.
Duct System
The pathway of saliva begins inside the lobule and ends in the oral cavity. As the secretion advances, the duct walls become thicker and the epithelium changes.
Intralobular ducts:
- Intercalated ducts. The smallest ducts with a narrow lumen. Lined by simple cuboidal basophilic epithelium. Externally, like acini, they are enveloped by a layer of myoepithelial cells.
- Striated ducts. These are continuations of the intercalated ducts. They are larger with a wide lumen. The epithelium is simple columnar with oxyphilic (pink) cytoplasm. A hallmark feature is basal striation, caused by deep infoldings of the plasmalemma and mitochondria oriented perpendicularly within them. Myoepitheliocytes are also found here.
Interlobular and main ducts: Exiting the lobules into the connective tissue septa. The simple epithelium first transitions to stratified cuboidal, and near the orifice (opening into the oral cavity) becomes stratified squamous. The main duct possesses its own connective tissue adventitia.
Comparative Characteristics of Major Glands
The histological picture of the glands changes predictably. The closer a gland is to the floor of the oral cavity, the more mucous component it contains and the more reduced its intercalated ducts are (during embryogenesis, their cells undergo mucous transformation and turn into mucocytes).
| Gland | Lobules | Terminal Units | Ducts | Secretion |
|---|---|---|---|---|
| Parotid (glandula parotidea) | 7 | Serous (protein) only | Intercalated and striated are equally well-developed | Purely proteinaceous |
| Submandibular (glandula submandibularis) | 10 | Serous and mixed (with demilunes) | Striated are more numerous, some intercalated transformed into mucous | Mixed (predominantly protein) |
| Sublingual (glandula sublingualis) | 18-20 | Mixed, mucous, rarely serous | Intercalated are nearly absent, mainly striated visible | Mixed (predominantly mucous) |