General structure
From a histological standpoint, all major salivary glands are classified as complex branched. This means that both their duct system branches ("complex") and their secretory units branch ("branched").
The gland consists of two main components:
- Stroma (supporting framework). Externally, the organ is covered by a dense connective tissue capsule. From it, septa extend inward, dividing the gland into lobules. Blood vessels and interlobular excretory ducts run within these layers.
- Parenchyma. Includes secretory (terminal) units where saliva is produced, and the duct system through which it is excreted into the oral cavity.
Terminal portions (acini) and their cells
Depending on the secreted fluid, terminal portions are divided into three types. All of them contain myoepithelial (basket) cells — flattened stellate structures of epithelial origin. They are located between the basement membrane and secretory cells, and upon contraction of their processes, they squeeze the secretion into the ducts.
- Serous (protein) units. Have an alveolar shape (rounded sac). Formed by serocytes — small pyramidal cells with a central round nucleus. Due to active protein synthesis, their cytoplasm is brightly basophilic (appears dark under a microscope).
- Mucous units. Have a tubular shape. Consist exclusively of mucocytes. These are large cells with light, "foamy" cytoplasm (storing mucus) and a flattened nucleus displaced toward the basal pole.
- Mixed (protein-mucous) units. Represent widened tubules. The central part is formed by light mucous cells, while a second layer of protein cells is located at the periphery. This protein "cap" is called a serous demilune (Gianuzzi's demilune). The protein secretion from the demilune flows into the lumen of the unit via special intercellular canaliculi.
System of excretory ducts
The pathway of saliva begins within the lobule and ends in the oral cavity. As the secretion moves forward, the duct wall thickens and the epithelium changes.
Intralobular ducts:
- Intercalated ducts. The smallest, with a narrow lumen. Lined with simple cuboidal basophilic epithelium. Outwardly, like acini, they are surrounded by a layer of myoepithelial cells.
- Striated ducts. They are a continuation of intercalated ducts. They are larger with a wide lumen. The epithelium is simple columnar with oxyphilic (pink) cytoplasm. A characteristic feature is basal striation, which results from deep folds of the plasmalemma and mitochondria aligned perpendicularly within them. Myoepithelial cells are also found here.
Interlobular and main ducts: Extend beyond the lobules into the connective tissue septa. The simple epithelium first becomes stratified, then stratified cuboidal, and right at the mouth (upon exit into the oral cavity) transitions into stratified squamous. The main duct has its own connective tissue sheath.
Comparative characteristics of major glands
The histological picture of the glands changes systematically. The closer the gland is to the floor of the oral cavity, the more mucous component it contains and the more reduced the intercalated ducts are (during embryogenesis, their cells undergo mucous metaplasia and transform into mucocytes).
| Gland | Lobules | Terminal units | Ducts | Secretion |
|---|---|---|---|---|
| Parotid (glandula parotidea) | 7 | Serous (protein) only | Intercalated and striated are equally well developed | Purely protein |
| Submandibular (glandula submandibularis) | 10 | Serous and mixed (with demilunes) | More striated ducts, some intercalated ones have undergone mucous metaplasia | Mixed (more protein) |
| Sublingual (glandula sublingualis) | 18-20 | Mixed, mucous, rarely serous | Intercalated ducts are almost absent, mainly striated ones are visible | Mixed (more mucus) |
It is easy to remember the direction of secretion using the "top-to-bottom" rule: from the ear (Parotid — pure protein, watery saliva) to the jaw (Submandibular — mixed, but more protein) and to the floor of the mouth (Sublingual — mixed, viscous mucus). The proportion of mucocytes increases, and intercalated ducts disappear!
Class and exam questions
Why is the cytoplasm of serocytes dark (basophilic), while that of mucocytes is light?
Enzymes (proteins) are actively synthesized in serocytes, which is why they have a powerfully developed protein-synthesizing apparatus that stains with basic dyes. In mucocytes, however, mucus (glycoproteins) accumulates, which under standard staining appears as light, empty "foam".
Where are myoepithelial cells located?
They are located between the basement membrane and the basal pole of secretory cells (in all types of acini), as well as around intercalated and striated intralobular excretory ducts.
How does the nervous system affect the composition of saliva?
Parasympathetic stimulation leads to the production of a large volume of thin (watery) saliva. Sympathetic innervation, conversely, stimulates the release of a small amount of thick, viscous saliva.
Where do the excretory ducts of the major glands open?
The parotid duct opens into the vestibule of the mouth at the level of the second upper molar. The ducts of the submandibular and sublingual glands open on the sublingual caruncle (while some of the minor ducts of the sublingual gland open independently).
What else to study on this topic
- Embryogenesis of salivary glands and the process of mucous metaplasia of primordia
- Chemical composition of saliva: mucin, enzymes, and biologically active substances
- Excretory function of salivary glands (excretion of nitrogenous wastes and ions)
- The phenomenon of basal striation of ducts: electron microscopic structure
- Analysis of gland histological specimens stained with hematoxylin and eosin
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