Parotid gland histology 5m answer
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Parotid Gland - Histology
Overview
The parotid is the largest of the three paired major salivary glands. It is a pure serous gland - the only major salivary gland composed entirely of serous acini, with adipose tissue distributed throughout its parenchyma (amount increases with aging).
1. Capsule and Stroma
- Enclosed by a dense fibrous capsule derived from the investing layer of deep cervical fascia
- Septa from the capsule divide the gland into lobules
- Stroma contains connective tissue, blood vessels, lymphatics, and nerves
- Characteristically, the parotid contains 10-20 intraglandular and periglandular lymph nodes - unique among the major salivary glands; the submandibular and sublingual glands have none. This is clinically important as many parotid masses represent lymph node metastases from primary head and neck skin cancers.
2. Acini (Secretory End-Pieces)
- Composed entirely of serous acinar cells
- Cells are pyramidal in shape, with a basally placed nucleus and secretory (zymogen) granules at the apex facing the acinar lumen
- Granules are periodic acid-Schiff (PAS)-positive and appear as blue-purple on H&E
- The cytoplasm is deeply basophilic at the base (due to abundant rough ER for protein synthesis) and eosinophilic/granular apically
- Secretion is watery and protein-rich (contains amylase, lysozyme, IgA)
- Protein synthesis pathway within acinar cells: rough ER → Golgi complex (post-translational modification) → secretory granules → exocytosis into acinar lumen
3. Myoepithelial Cells
- Located between the basal lamina and the basal surface of acinar cells (and intercalated duct cells)
- Stellate or basket-shaped cells surrounding the acini
- Contain both smooth muscle and epithelial characteristics; rich in ATP
- Have a contractile function - squeeze the acini and ducts to propel secretion toward the duct system
- Not readily visible on routine H&E; best seen with immunostains for smooth muscle actin
4. Duct System (in order, from acinus outward)
The basic secretory unit is the salivon = acinus + intercalated duct + striated duct + excretory duct.
| Duct | Epithelium | Function |
|---|---|---|
| Intercalated duct | Simple cuboidal epithelium; smallest diameter | Drains acinus; myoepithelial cells present along abluminal side |
| Striated duct (intralobular) | Simple columnar; distinct basal striations from membrane infoldings | Active reabsorption of Na⁺ and Cl⁻, secretion of K⁺ and HCO₃⁻ - modifies salivary composition |
| Excretory duct (interlobular/main) | Stratified cuboidal to columnar; surrounded by connective tissue | Conveys saliva to Stensen's duct opening at the parotid papilla opposite the upper 2nd molar |
- The basal striations of striated duct cells are formed by deep infoldings of the basal plasma membrane containing numerous mitochondria oriented perpendicular to the base - the ultrastructural basis of active ion transport
- Both intercalated and striated ducts can modify salivary composition
5. Comparison with Other Major Salivary Glands
| Feature | Parotid | Submandibular | Sublingual |
|---|---|---|---|
| Acinar type | Pure serous | Mixed (predominantly serous, ~10% mucous; serous demilunes) | Mixed (predominantly mucous; serous demilunes visible) |
| Intraglandular lymph nodes | Present (10-20) | Absent | Absent |
| Adipose tissue | Present, increases with age | Less | Less |
Key Points for Exam
- Parotid = pure serous gland (no mucous acini)
- Serous cells: pyramidal, basal nucleus, apical PAS-positive granules
- Myoepithelial cells surround acini and intercalated ducts; contractile function
- Striated duct = active ion transport; basal striations = membrane infoldings + mitochondria
- Parotid uniquely contains intraglandular lymph nodes (clinically significant for metastases)
- Adipose tissue in parotid parenchyma increases with aging (acinar cells are lost and replaced by fat)
Sources: Histology: A Text and Atlas with Correlated Cell and Molecular Biology, p. 1492; Cummings Otolaryngology Head and Neck Surgery, p. 1530; KJ Lee's Essential Otolaryngology, p. 647; Scott-Brown's Otorhinolaryngology Head & Neck Surgery
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