Parotid gland

The parotid gland is the largest of the three major salivary glands. It is responsible for producing serous (watery) saliva, which contains enzymes like amylase that aid in the initial digestion of carbohydrates.^[6] The parotid gland has a significant role in maintaining oral moisture and facilitating chewing and swallowing.

Location

The parotid gland is located anterior to the ear, extending from the zygomatic arch (cheekbone) to the angle of the mandible (lower jaw).^[5] It lies superficial to the masseter muscle, wrapping around the posterior part of the mandible. The duct of the parotid gland, known as the Stensen’s duct, opens into the oral cavity near the second upper molar.

Anatomy

The parotid gland is the largest of the three major salivary glands, located in the region anterior to the ear. It plays a central role in the production of serous (watery) saliva. Below is a detailed description of its anatomy:

Size and Shape

• The parotid gland is the largest salivary gland, weighing about 15–30 grams in adults.
• It is triangular in shape, with the base facing the ear and the apex directed downward toward the angle of the jaw.

Capsule

• The gland is encapsulated by a dense fibrous capsule, which is continuous with the parotid fascia.
• The capsule is important for protecting the gland’s delicate structures, though it can also limit its expansion in cases of swelling.^[7]

Structure

• The parotid gland consists of lobules formed by clusters of acinar cells, which produce saliva.
  • These acinar cells are organized into serous acini, which secrete watery saliva rich in enzymes like amylase.^[4]
• The gland is divided into superficial and deep portions by the facial nerve (cranial nerve VII):
  • Superficial lobe: Larger portion, located anterior and inferior to the ear.
  • Deep lobe: Smaller portion, located between the ramus of the mandible and the styloid process of the temporal bone.

Duct System

The excretory duct of the parotid gland is known as Stensen’s duct:

• It originates from the deep lobe of the parotid gland.
• The duct passes horizontally, over the masseter muscle, and enters the buccal mucosa (the lining of the cheek).
• It opens into the oral cavity opposite the second upper molar at the parotid papilla.

Blood Supply

• The parotid gland is primarily supplied by the superficial temporal artery, a branch of the external carotid artery.
• The transverse facial artery, also a branch of the external carotid, supplies the lateral aspect of the gland.

Venous Drainage

Venous drainage from the parotid gland is through the retromandibular vein, which drains into the internal jugular vein.^[8]

Nerve Supply

• Parasympathetic Innervation:
  • The parasympathetic fibers arise from the glossopharyngeal nerve (cranial nerve IX).
  • The preganglionic fibers synapse at the otic ganglion, from where postganglionic fibers travel via the auriculotemporal nerve to stimulate saliva production.
• Sympathetic Innervation:
  • Sympathetic fibers come from the superior cervical ganglion and modulate the saliva flow, often reducing secretion during stress.^[3]
• Sensory Innervation:
  • Sensory fibers for pain and temperature sensations come from the auriculotemporal nerve, a branch of the mandibular nerve (CN V3).

Lymphatic Drainage

Lymph from the parotid gland drains into the preauricular lymph nodes and then to the deep cervical lymph nodes.

Relations

• Anteriorly:
  • The masseter muscle and buccal fat pad.
  • The facial artery and facial nerve are found in the anterior region.
• Posteriorly:
  • The external ear and sternocleidomastoid muscle.
• Medially:
  • The styloid process and the internal carotid artery.

Histology

• The parotid gland consists primarily of serous acini that secrete watery, enzyme-rich saliva.
• The intercalated ducts and striated ducts help modify the saliva, adding electrolytes and fluid to maintain proper composition for digestion and lubrication.

Function

The parotid gland is the largest of the three major salivary glands and plays a crucial role in oral health, digestion, and the maintenance of the oral environment. Its function is primarily focused on producing and secreting saliva. Below is a detailed explanation of the functions of the parotid gland:

Saliva Production

The primary function of the parotid gland is to produce serous (watery) saliva:

• This saliva is rich in enzymes, particularly amylase, which begins the process of carbohydrate digestion by breaking down starches into simpler sugars such as maltose and dextrins.
• The gland also produces a small amount of lysozyme, an antimicrobial enzyme that helps protect the oral cavity from bacterial infections.

Moisturizing and Lubricating the Oral Cavity

The saliva secreted by the parotid gland helps to moisten and lubricate the oral mucosa:

• This lubrication facilitates speech by preventing dryness and ensuring smooth tongue and lip movements.
• It also makes chewing easier by keeping the mouth moist, preventing friction between teeth and soft tissues during mastication.

Facilitating Swallowing

The parotid gland’s secretion contributes to the formation of a bolus, a soft mass of chewed food mixed with saliva:

• This bolus is easier to swallow and moves more efficiently from the oral cavity into the esophagus.
• Saliva also serves as a medium for dissolving food molecules, which aids in the initial stages of taste perception.

Protection of Teeth and Oral Tissues

Saliva from the parotid gland helps maintain oral hygiene and prevent tooth decay:

• The bicarbonate ions in saliva help neutralize acids produced by oral bacteria, reducing the risk of dental caries.
• Lysozymes and immunoglobulin A (IgA) found in saliva protect against bacterial colonization, helping to prevent oral infections.

Supporting Taste Perception

The saliva secreted by the parotid gland aids in the dissolution of food particles, allowing them to interact with taste receptors on the tongue:

This enhances taste sensation by allowing food molecules to reach the taste buds more efficiently.

Regulation of pH in the Mouth

The parotid gland helps regulate the pH balance in the mouth:

• The saliva neutralizes acidic foods or beverages and balances the pH, preventing damage to the enamel of the teeth.
• This buffering function protects against conditions such as acid reflux or erosion of tooth enamel.

Fluid and Electrolyte Balance

Saliva from the parotid gland is composed of water, electrolytes (sodium, potassium, chloride, bicarbonate), and proteins:

• This composition helps maintain oral hydration and contributes to the body’s overall fluid and electrolyte balance.
• It aids in the hydration of the oral cavity, preventing excessive dryness, especially during periods of low salivation.

Antimicrobial Defense

The antimicrobial properties of saliva secreted by the parotid gland help to defend the oral cavity against pathogens:

• Lysozyme and lactoferrin found in parotid saliva help break down bacterial cell walls and inhibit bacterial growth.^[2]
• IgA antibodies help neutralize viruses and bacteria, contributing to oral immune defense.

Facilitating Digestion

Amylase in the saliva begins the digestion of carbohydrates, a process that continues in the stomach and small intestine:

By initiating starch breakdown in the mouth, the parotid gland plays an early role in the overall digestive process.

Lubricating the Pharynx and Esophagus

The parotid gland’s saliva helps lubricate the pharynx and esophagus, aiding in the smooth passage of the bolus of food to the stomach during swallowing.

Clinical significance

The parotid gland plays an important role in oral health and digestion, and its dysfunction can lead to several clinical conditions:

• Sialolithiasis (Salivary Stones): The parotid gland is a common site for the formation of salivary stones, which can block the duct (Stensen’s duct), causing swelling, pain, and difficulty in swallowing.^[1]
• Sialadenitis: Infections or inflammation of the parotid gland, often caused by bacterial infections, dehydration, or viral infections such as mumps, can lead to swelling, pain, and fever.
• Parotid Tumors: Benign tumors such as pleomorphic adenomas or malignant tumors like mucoepidermoid carcinoma can develop in the parotid gland, leading to facial asymmetry, pain, or difficulty swallowing.
• Dry Mouth (Xerostomia): Dysfunction of the parotid gland, often due to autoimmune conditions like Sjögren’s syndrome, radiation therapy, or medication side effects, can lead to reduced saliva production, resulting in dry mouth, swallowing difficulties, and increased risk of dental caries.
• Facial Nerve Injury: The facial nerve (cranial nerve VII) runs through the parotid gland, and any surgery or trauma in the parotid region can risk damaging the nerve, leading to facial paralysis or weakness on the affected side.