**2. Classification, structure, and anatomy of salivary glands**

	- **a.** Major salivary glands
		- Parotid
		- Submandibular
		- Sublingual

## **b.** Minor salivary glands

	- **i.** Anterior lingual gland (glands of Blandin and Nuhn)
	- **ii.** Posterior lingual serous gland (von Ebner's glands)
	- **iii.** Posterior lingual mucous gland
	- Serous
	- Mucous

gland and divided into two main groups. The major salivary glands include the paired parotid, submandibular, and sublingual glands. Additionally, the mucosa of the upper aerodigestive tract is lined by hundreds of small, minor salivary glands. The connective tissue forms a capsule around the gland and extends into it, dividing groups of secretory units and ducts into lobes and lobules. Blood vessels, lymphatic vessels and nerves that

The salivary glands are compound glands as they have more than one tubule entering the main duct, and the architectural arrangement is tubuloacinar, where acini are secretory units. These secretory units are merocrine as they release only the secretion of the cell from the secreting units. Myoepithelial cells are contractile cells associated with the secretory end-

supply the gland, are present within the capsule [1].

**Figure 1.** Structural organization of salivary gland [3].

64 Histology

pieces and intercalated ducts of the salivary gland [2].

**a.** Major salivary glands

○ Submandibular

○ Sublingual

○ Parotid

**2. Classification, structure, and anatomy of salivary glands**

• On the basis of size and location, salivary glands are classified as [1, 2]:

#### **2.1. Major salivary glands**

These are the largest, bilaterally paired, and situated extraorally, but their secretion reaches the oral cavity by variable long ducts.

#### *2.1.1. Parotid gland*

The parotid gland is the largest of all the salivary glands and weighs about 15–30 g. It is located below the external acoustic meatus between the ramus of the mandible and the sternocleidomastoid. It is divided by facial nerve into a superficial and deep lobe. The superficial lobe, overlying the lateral surface of the masseter, is defined as the part of the gland lateral to the facial nerve. The deep lobe is medial to the facial nerve and located between the mastoid process of the temporal bone and the ramus of the mandible. An accessory parotid gland may also be present lying anteriorly over the masseter muscle between the parotid duct and zygomatic arch [4].

The parotid duct, also known as Stensen's duct, secretes serous saliva and opens into the vestibule of the mouth (gingiva-buccal vestibule) opposite the crown of the upper second molar tooth [2].

#### *2.1.2. Submandibular gland*

It is the second largest salivary gland, also known as submaxillary salivary gland, weighs about 7–16 g and is almost the size of a walnut. It is situated in the submandibular triangle, which has a superior boundary formed by the inferior edge of the mandible and inferior boundaries formed by the anterior and posterior bellies of the digastric muscle. The gland is approximately J-shaped being indented by the posterior border of the mylohyoid which divides into a larger part superficial to the muscle and a smaller part lying deep to the muscle [4]. The submandibular gland duct, also known as Wharton's duct, is thin-walled, about 5 cm long, and runs forward above the mylohyoid muscle lying just below the mucosa of the floor of the mouth in its terminal portion. The duct opens on the floor of the mouth, on the summit of the sublingual papilla also called the caruncula sublingualis, lateral to the lingual frenulum [2].

**3. Development of the salivary glands**

of the supporting part of the adult gland.

glands.

12th prenatal week.

Stages of development [5, 6]

out the development.

bulbs.

The development of the glandular tissue involves the interaction of the epithelium with the underlying mesenchyme to form the functional part of the tissue [5, 6]. These epithelialmesenchymal interactions are also known as secondary induction in which the mesenchyme is in close proximity with the epithelium and is required for the normal development of the epithelium. For example, epithelial-mesenchymal interactions regulate both the initiation and growth of the glandular tissue and the eventual cytodifferentiation of cells within the salivary glands. The mesenchyme, therefore, is required for normal development as well as formation

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All salivary glands follow a similar development pattern. The functional glandular tissue (parenchyma) develops as an epithelial outgrowth (glandular bud) of the buccal epithelium that invades the underlying mesenchyme. The connective tissue stroma (capsule and septa) and blood vessels form from the mesenchyme. The mesenchyme is composed of cells derived from neural crest and is important for the normal differentiation of the salivary

As the bud formation begins during development, the portion of the bud closest to the stomodeum eventually differentiates into the main excretory duct of the gland, while the most distal portion of the bud forms the secretory endpieces or acini. The origin of the epithelial buds is believed to be ectodermal in the parotid and minor salivary glands but endodermal in origin in the submandibular and sublingual glands. The parotid gland originates near the corners of the primitive oral cavity by the sixth week of prenatal life. The submandibular glands arise from the floor of the mouth at the end of the sixth or the beginning of the seventh week in utero. The sublingual gland forms lateral to the submandibular primordium at about eighth week. All the minor salivary glands bud from buccal epithelium but start after their

**I. Bud formation, i.e., induction of oral epithelium by underlying mesenchyme:** The mesenchyme underlying the oral epithelium induces the proliferation in the epithelium

**II. Formation and growth of epithelial cord:** A solid cord of cells forms from the epithelial bud through cell proliferation. Condensation and proliferation occur in the surrounding mesenchyme which is closely associated with the epithelial cord. The basal lamina plays a role in influencing morphogenesis and differentiation of the salivary glands through-

**III. Initiation of branching in terminal parts of epithelial cord and continuation of glandular differentiation:** The epithelial cord proliferates rapidly and branches into terminal

**IV. Dichotomous branching of epithelial cord and lobule formation:** The branching continues at the terminal portion of the cord forming an extension treelike system of bulbs. As branching occurs, the connective tissue differentiates around the branches, eventually

which results in tissue thickening and bud formation.

#### *2.1.3. Sublingual gland*

It is the smallest of all the three major salivary glands that is almond shaped and weighs about 3–4 g. The gland lies above the mylohyoid, below the mucosa of the floor of the mouth, medial to the sublingual fossa of the mandible, and lateral to the genioglossus [4]. It comprises of one main gland duct with various small ducts. The main duct, Bartholin's duct, opens with or near the submandibular duct. Several smaller ducts, duct of Rivinus, open independently along the sublingual fold [2].

#### **2.2. Minor salivary glands**

The minor salivary glands are placed below the epithelium in almost all parts of the oral cavity. These glands comprise numerous small groups of secretory units opening via short ducts directly into the mouth. They lack a distinct capsule, instead mixing with the connective tissue of the submucosa or muscle fibers of the tongue or cheeks [2].

#### *2.2.1. Labial and buccal glands*

These glands are present on the lips and cheeks and comprise of mucous tubules with serous demilunes [1, 2].

#### *2.2.2. Glossopalatine glands*

These are located to the region of the isthmus in the glossopalatine fold but may extend from the posterior extension of the sublingual gland to the glands of the soft palate [1, 2].

#### *2.2.3. Palatine glands*

These are located in the glandular aggregates present in the lamina propria of the posterolateral aspect of the hard palate and in the submucosa of the soft palate and uvula [1, 2].

#### *2.2.4. Lingual glands*

The glands of the tongue can be divided into various groups [1, 2]. The anterior lingual glands (glands of Blandin and Nuhn) are present near the apex of the tongue. The ducts open on the ventral surface of the tongue near the lingual frenulum. The posterior lingual mucous glands are present lateral and posterior to vallate papillae and in association with lingual tonsil. The ducts of these glands open on the dorsal surface of the tongue. The posterior lingual serous glands (von Ebner's glands) are located between the muscle fibers of the tongue below the vallate papillae, and the ducts open into the trough of circumvallate papillae and at the rudimentary folate papillae on the sides of the tongue.
