Salivary glands - Structure (Note & Video)

This video covers the classification, structure and the histology of the secretory cells and ducts of the salivary gland. I have split the whole video topic into two notes - one covering it's classification and structure and the other explaining the histology of the secretory cells and ducts.

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• Salivary glands (SG) are compound, exocrine glands whose function is to secrete saliva.

• Saliva, 90% of which is secreted by major salivary glands (which we shall discuss subsequently), is a multifunctional fluid, helping in maintaining the integrity of the oral cavity in various ways.

💡FUNCTIONS OF SALIVA

- The mucin in the saliva plays a major role in lubrication. This helps in prevention of oral tissues adhering to one another (keeps tissue moist). As a whole this helps in mastication, deglutition(swallowing) and speech.
- The buffering action of saliva, as a result of the presence of bicarbonate and phosphate ions, help in maintaining integrity of the teeth (protects from demineralization, promotes remineralization if demineralized).
- Proteins like lactoferrin and lysozyme in saliva have anti-bacterial action.
- Saliva helps in food digestion (amylase and lipase).
- Immunoglobulin A (IgA) is actively secreted in saliva. It contributes to mucosal immunity.

CLASSIFICATION OF SALIVARY GLANDS

SGs can be classified according to their size and the secretory cells they posses.

According to size

• Major salivary glands

• Minor salivary glands

Major salivary glands comprise 3 pairs of glands them being,

• Parotid glands

• Submandibular glands

• Sublingual glands

Minor salivary glands are scattered throughout the oral cavity and are named according to their location.

• Labial glands

• Buccal glands

• Palatal glands

• Lingual glands (von Ebner’s glands)

• Glossopalatine glands

• Retromolar glands

💡KNOW THY FACTS

- The minor salivary glands are at least 600-1000 in number.
- The minor glands only contribute 10% of the total salivary secretion.
- Most minor salivary glands are predominantly mucous glands, with the exception of von Ebner's glands that are purely serous.
- Minor glands are not found on the gingiva and anterior hard palate.

According to type of “secretory cell”

SGs can be

• Purely serous

• Purely mucous

• Mixed serous-mucous with either of the secretory cell predominating.

- 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.

STRUCTURE OF SALIVARY GLANDS

Secretory cells

• SGs are made of secretory units called “acini“ (singular - acinus), that house secretory cells.

• Secretory cells are of two types - serous and mucous secretory cells.

• Since secretory (serous and mucous) cells are housed in an acinus, the cells are also collectively called “acinar cells“.

• The secretory units communicate with the oral cavity through a complex ductal arrangement.

• The secretory cells (acinar cells) are responsible for secreting the primary salivary secretion.

Ductal arrangement

• Secretory units open into an “intercalated duct“, which is continuous with the “striated duct“, which in turn opens into the “excretory duct“ of the salivary gland.

• The ducts like the secretory units, are lined by cells called ductal cells.

• The saliva secreted by acinar cells pass through the ducts to be excreted finally via the excretory duct into the oral cavity.

• On it’s way, the saliva’s composition is influenced or affected by the intercalated and striated ducts.

Myoepithelial 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.

• The main function of the myoepithelial cell is to contract and help squeeze out secretory products from the secretory end pieces to the ducts and finally to the oral cavity.

💡KNOW THY FACTS

This architecture of the salivary glands can be compared to a bunch of grapes, where the grapes represent the secretory end pieces (acini) and the stem/branches represent the ducts.

LOBULAR ARCHITECTURE

• Salivary glands are basically enclosed by a connective tissue capsule that branches 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.

A NOTE ON ONCOCYTES

• Oncocytes are cells that are packed with mitochondria and appear granular and deep pink under the microscope.

• Oncocytes could appear in the acini and ducts of normal salivary glands.

• The number of oncocytes in the salivary gland increase with age.

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HIGHLIGHTS - VIVA & ENTRANCE EXAM PERSPECTIVE

• SGs are made of secretory units called “acini“ (singular - acinus), that house secretory cells.

• Secretory cells are of two types - serous and mucous secretory cells.

• Since secretory (serous and mucous) cells are housed in an acinus, the cells are also collectively called “acinar cells“.

• Secretory units open into an “intercalated duct“, which is continuous with the “striated duct“, which in turn opens into the “excretory duct“ of the salivary gland.

• The ducts like the secretory units, are lined by cells called 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.

• Major salivary glands comprise 3 pairs of glands them being, -> a) Parotid glands, b) Submandibular glands, c) Sublingual glands.

• 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.

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REFERENCES AND FURTHER READING

1. Berkovitz BKB, Hollan GR, Moxham BJ. Oral Anatomy, Histology and Embryology. 4^th ed. Mosby Elsevier; 2009.

2. Nanci A. Tencate’s Oral Histology. Development, Structure and Function. 8^th ed. Elsevier; 2013.

3. Kumar GS. Orban’s Oral Histology and Embryology.13^th ed. Elsevier; 2011.

4. Avery JK. Oral development and Histology. 3^rd ed. Thieme Medical Publishers; 2002.