Parotid gland

Gross anatomy 

Each individual is gifted with a pair of parotid glands that are situated in the preauricular area (flattened region anterior to the tragus) on each side of the face. It is the largest of the three major salivary glands (the others being the submandibular and sublingual glands), and weigh around 15 – 30 grams each. Each gland is irregular, consisting of a superficial and a deep lobe, and is tan-yellow in appearance. Although the parotid gland is described as being irregular, its general outline is that of an inverted pyramid. The base is the most superior part of the gland, while the blunted apex points inferiorly. It also has anteromedial, posteromedial, and superficial surfaces.

Borders and surfaces

The borders of the parotid gland are as follows:

• Superiorly by the zygomatic arch 
• Posterosuperiorly by the external acoustic meatus and the ramus of the mandible 
• Posteroinferiorly by the mastoid process and distal part of the mastoid head of sternocleidomastoid muscle
• Inferiorly the gland is limited by the sternocleidomastoid muscle
• Anteriorly the masseter muscle forms another boundary 

The superior surface of the gland is covered by the investing layer of the deep cervical fascia, posterior fibers of the platysma, neighboring lymphatic tracts, fibers of the great auricular nerve (anterior ramus of spinal nerves C2  and C3), and skin. The deep surface of the gland is related to the buccinator muscle.

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vascular supply

The arteries and veins that traverse the substance of the parotid gland are also responsible for supplying and draining the organ. The arterial supply arises from branches of the external carotid artery, while the venous drainage is via the retromandibular vein. Lymphatic fluid drains to the superior deep cervical lymph nodes. 

Innervation

Every structure in the human body have interactions with the brain through any varying combinations of cranial and peripheral nerves. There are two major modalities that are transmitted to and from the parotid gland - autonomic instructions and sensory perceptions. These nerve impulses are transmitted along cranial nerves to the gland. 

So, which cranial nerve innervates the parotid salivary gland? The gland is under parasympathetic regulation. It receives these fibers via the lesser petrosal nerve, which is a branch of the glossopharyngeal nerve (CN IX). Of note, CN IX synapses on the otic ganglion. The postganglionic secretomotor fibers that emerge from the otic ganglion reach the parotid gland via the auriculotemporal nerve (a branch of the mandibular division of the trigeminal nerve [CN V3]). The sympathetic supply is derived from the adjacent sympathetic plexus of the carotid sheath. The auriculotemporal nerve is also responsible for carrying general visceral afferent (sensory) stimuli from the gland as well. 

Remember the secretomotor pathway of the parotid gland with the mnemonic "IT has Lesser Options Anywhere", which stands for;

• Inferior salivatory nucleus
• Tympanic branch of 9th nerve
• Lesser petrosal nerve
• Otic ganglion
• Auriculotemporal nerve

Histology

As an exocrine gland, the parotid gland is composed of a lobular system of branching ducts that are separated by connective tissue septa. Like its fellow major salivary glands, the parotid gland is also surrounded by a dense connective tissue capsule. It also has a pseudocapsule arising from the deep investing layer of the cervical fascia. It produces watery, serous saliva as opposed to seromucous secretions like the other two major salivary glands, or mucous secretions like the minor glands. 

The parotid gland is rich in serous cells, which are pyramidal cells with circular nuclei. These cells are polarized and rich in rough endoplasmic reticulum and secretory granules towards the apex of the cells. The cells are tightly adhered to each other at the respective tight junctions and are arranged in grape-like clusters known as serous acini (singular, “acinus”). They are also rich in alpha-amylase, which promotes hydrolysis of carbohydrates and protein abundant in proline.

The tubules and ductules leading from the acini also contain flattened, small, myoepithelial cells. These cells are responsible for the contractile activity of the ducts as they express the stored saliva. Smaller ducts are known as intralobular ducts, and they drain into the intercalated ducts. The walls of the intercalated ducts contain cuboidal epithelial cells. Intercalated ducts then unite to form striated ducts, which contain columnar cells in the walls and highly folded basolateral membranes. It is the rich population of mitochondria lined along the basolateral membrane that gives the impression of striations along the cells. The surrounding connective tissue is populated with plasma cells that release immunoglobulin A (IgA). IgA is bound to products in the serous acini and intralobular ducts so that they can be released into the saliva. This provides additional protection for the host organism against pathogens that may have entered the oral cavity.

Finally, ducts arising from each lobule combine to form excretory ducts. The walls of these larger conduits contain more connective tissue, and an atypical arrangement of simple columnar or cuboidal, stratified columnar or cuboidal, or pseudostratified epithelia.Are you curious to discover more about the histological structure of the parotid glands?

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Embryology

The parotid glands are the first of the three paired major salivary glands to start forming. The process begins in the 6th gestational week with a primitive scaffold or primordial anlage of the parotid gland that acts as a scaffold for future gland development. The scaffold arises from the posterior part of the stomodeum that is composed of ciliated epithelial cells in the lumen, and ectodermal myoepithelial cells externally. The primordial gland grows laterally as the mandible continues to elongate, and it forms solid cords over the surface of the masseter muscle as it develops.

By the 10th gestational week, there is numerous arborization and canalization resulting in a lobulated appearance of the developing parotid gland. As the gland continues to grow, it is encapsulated by surrounding mesenchyme (which goes on to form the connective tissue capsule that surrounds the gland). The terminal segments of the developing cords give rise to the formation of acini (singular, “acinus”) and production of glandular secretions (e.g. saliva) usually by the beginning of the 18th gestational week. An acinus is a Latin word that means “berry with seeds,” like grapes. It is used to describe the clustered appearance of the cells within the gland.

The process of arborization and elongation of the acini and glands continues into the 28th gestational week, where more primitive acini and duct systems typical of glands continue to form. This stage is strongly linked to programmed cell death (apoptosis) of the centrally located cells and the addition of more cells at the proximal and distal ends of the tube. Further maturation of these acini and duct pathways is completed during the later stage of gland development.

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