The oral cavity is an important point of entry for both the respiratory and digestive tracts. It also houses the tongue, which plays an integral role in speech, ingestion, and swallowing. Due to the oral cavity’s constant exposure to the external environment, it is easy for the organs within the mouth to get dry. In addition to ingesting fluids, the body also releases a modest amount of secretions to help with keeping the area moist, as well as facilitating in the digestive process. The salivary glands aid in this process by secreting saliva, which has protective, digestive, and lubricating properties. Salivary glands are a subtype of exocrine glands, which are glandular structures that involve a duct system to release their products onto an epithelial surface. This differs from endocrine glands (like the adrenal and thyroid glands) that release their products directly into the bloodstream.
There are two sets of salivary glands within the oral cavity responsible for these secretions: the major and minor salivary glands. The major salivary glands are all paired and consist of the parotid, submandibular, and sublingual glands. They are encapsulated and have their own ducts through which saliva is secreted into the buccal cavity.
The minor salivary glands are numerous (ranging between 600 to 1000 glands), are mostly unnamed, and are widely distributed throughout the oral cavity. Those that are named include the labial, buccal, molar, palatine, and lingual glands. Unlike the major salivary glands, the minor glands are not encapsulated and they either share duct systems with adjacent salivary glands or have their own
|Gross anatomy||Largest of the salivary glands
Irregular in shape, but roughly pyramidal
Divided into superficial and deep lobes
The facial nerve (CN VII), external carotid artery, retromandibular vein all pierce the parotid gland
Drains via the parotid duct of Stensen
Accessory parotid gland related superiorly to the duct
|Borders||Superior – zygomatic arch
Posterior – external acoustic meatus and sternocleidomastoid muscle
Anterior – sternocleidomastoid muscle inferiorly, masseter muscle
|Neurovascular supply and lymphatic drainage||Arterial Supply – superficial temporal artery, maxillary artery, transverse facial artery branches of the external carotid artery
Venous Drainage – retromandibular vein
Lymphatic Drainage – deep cervical lymph nodes
Innervation – glossopharyngeal nerve (CN IX) and auriculotemporal nerves
|Embryology||Ectodermal myoepithelial origin
6th gestational week – development begins
10th gestational week – arborization and canalization begins
18th gestational week – early secretions
28th gestational week – the maturation of the acini and ducts
|Parotid gland disorders||Sialolithiasis (parotid gland stones)
This article will focus on the parotid gland. Discussions will include information about its embryology, gross anatomy and related structures, histology, neurovascular supply, and lymphatic drainage. There are several diagrams throughout the article which will also help in understanding the anatomy of the gland. Additional clinical discussions about pathological processes associated with the parotid gland will also be included.
- Gross anatomy
- Borders and surfaces
- Vascular supply
- Disorders of the parotid gland
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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Structures passing through the parotid gland
The gland is separated into the superficial and deep lobes by the facial nerve (CN VII). The nerve enters the posterior border of the parotid gland after leaving the stylomastoid foramen. Within the substance of the gland, CN VII bifurcates into the superior (or temporofacial) and inferior (cervicofacial) divisions. The part of the gland that is medial to CN VII is the deep lobe, while the part that is lateral to the nerve is the superficial lobe.
Another important structure that passes through and divides within the substance of the parotid gland is the external carotid artery. As it passes through the posteromedial aspect of the gland, it gives off the maxillary artery, as well as the superficial temporal artery. Occasionally it also gives off the posterior auricular artery within the substance of the parotid gland as well.
The third structure that passes through the parotid gland is the retromandibular vein. It is the product of the union of the superficial temporal and maxillary veins. They enter the parotid gland at the same point that their corresponding arteries leave the gland. It passes caudally toward the apex of the gland, where it leaves posteriorly to divide into anterior and posterior branches. The former joins the facial vein, while the latter joins the posterior auricular vein.
Parotid duct (Stensen Duct)
Two major ducts arising from the parotid gland unite within the substance of the organ to form the parotid duct of Stensen. The parotid duct is roughly 7 cm x 3 mm long and leaves the superior part of the anteromedial surface of the gland. It passes horizontally over the surface of the masseter muscle then courses medially toward the anterior border of the muscle. The duct then makes an abrupt right turn to cross the buccinator muscle and associated buccal fat pad.
The parotid duct has a short submucosal course beginning at the crown of the upper third molar, then anteriorly and obliquely toward the upper second molar. The duct will eventually pierce the buccal mucosa to enter the oral cavity via a papilla adjacent to the upper second molar tooth. The aforementioned submucosal course provides a valvular mechanism to prevent reflux of air into the gland during instances of raised intraoral pressure (for example, while blowing out the cheeks).
Occasionally, there is an isolated accessory parotid gland which was historically referred to as the socia parotidis. It, along with the transverse facial artery, rests on the superior border of the parotid duct. The accessory duct gives off about 5 to 6 ductules that join the main parotid duct. The accessory duct is also superiorly related to the mandibular branch of the trigeminal nerve (CN V3). The buccal branch of the facial nerve (CN VII) also travels with the parotid duct as it goes to innervate the buccinator muscle.
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.
The branches of the external carotid artery that supply the parotid gland are the superficial temporal, maxillary, and (indirectly) the transverse facial arteries. The external carotid artery travels parallel to the mandible after leaving the carotid bifurcation. It continues toward the parotid gland to enter the substance of the organ. Here, it gives off the superficial temporal and maxillary arteries (which are both terminal branches of the external carotid artery).
The superficial temporal artery supplies the superior aspect of the gland, while the maxillary artery supplies the medial aspect of the gland. The transverse facial artery arises off the superficial temporal artery. Not only does it supply the parotid gland, but also the duct and nearby masseter muscle as well.
The maxillary and superficial temporal veins, which travel alongside their similarly named arteries, drain deoxygenated blood from the parotid gland. The veins unite to form the retromandibular vein, which traverses the substance of the parotid gland. As it leaves the gland, it branches into anterior and posterior counterparts.
The anterior division of the retromandibular vein descends towards the apex of the parotid gland, where it leaves the substance of the gland. It sometimes unites with the facial and lingual veins, to give rise to a common facial vein (which drains into the internal jugular vein). Similarly, the posterior branch of the retromandibular vein leaves the posterior border of the parotid gland. Here it may unite with the postauricular vein before draining into the external jugular vein.
There are numerous lymph nodes distributed throughout and around the substance of the parotid gland. This is an exception to the norm as all other salivary glands (both major and minor) do not have lymph nodes within the glandular tissue and have far fewer nodes surrounding them. The lymph nodes of the parotid gland are distributed throughout the superficial and deep lobes of the gland.
The majority of the lymph nodes (about 90%) are found in the superficial node. The nodes themselves are situated close to the surface of the gland, between the capsule and glandular tissue. The superficial set of lymph nodes drains the external acoustic meatus, auricle (pinna), scalp, eyelids, and lacrimal glands in addition to the parotid gland. The deep set of lymph nodes also drains other structures in addition to the parotid gland: external acoustic meatus, soft palate, middle ear, and nasopharynx.
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
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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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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Disorders of the parotid gland
Pathologies associated with the salivary glands may range from infectious to non-infectious inflammatory processes. Within the non-infectious class of disorders, there may also be a benign or neoplastic phenomenon occurring within the organ.
The key to unraveling disorders of the parotid gland is to ask the patient about a history of pain and swelling in the region of the gland and determining the relationship of pain and swelling to meals, as acute onset of pain on swelling with salivation is indicative of an obstructed duct. Some patients may complain about a lump in the gland. Determine if there has been any change in the size of the mass and over what period of time this has occurred. Also ask about constitutional symptoms (such as unintentional weight loss, night sweats, and low-grade fevers) as this may be suggestive of a non-benign process. Other important aspects include a history of dry mouth, a history of autoimmune disorders, and vaccination history. All of these factors are suggestive of a diffuse issue with the glands.
Parotid gland stones (Sialolithiasis)
Calculi or stones in the salivary gland are more common in the submandibular gland than the parotid gland. The presence of the stone obstructs the flow of saliva from the gland leading to acute swelling with meals. The swelling usually resolves about an hour after the onset of symptoms. Palpation of the duct or visualization of the duct orifice will likely reveal the offending agent. Plain radiography may also reveal the presence of the stones, as the majority of them are radio-opaque.
Parotid gland inflammation (Parotitis)
Both infectious and non-infectious inflammatory processes can affect the parotid glands. Historically, the mumps infection – caused by the mumps rubulavirus (a paramyxovirus) – was notorious for causing painful swelling of the parotid glands in childhood. Fortunately, this has become a historical teaching point with the advent of the mumps vaccine given with measles and rubella vaccinations during routine vaccination programs. Unfortunately, there are other pathogens that are able to incite an inflammatory response within the salivary glands. These include, but are not limited to:
- Human immunodeficiency virus (HIV)
- Granulomatous infections (i.e. tuberculosis)
The infectious processes usually result in bilateral manifestation of the disease, with painful swelling to both glands. There are usually systemic signs of infection such as erythema (redness around the area), malaise (generalized weakness), tenderness, and fever. These disorders are highly communicable and adequate precaution should be taken to limit their spread.
The duration of parotitis, as well as the choice of treatment, is dependent on the underlying cause of the inflammation. For example, if the cause of parotitis is an influenza infection, then the course of illness would be self-limiting as a healthy immune system will eradicate the virus over a few days. However, if the cause of the parotitis is related to the human immunodeficiency virus, then a cure will be dependent on control of the virus (i.e. compliance with antiretroviral medication).
This process is similar to the acute parotitis, with the exception that the offending agent is bacterial in origin. It may also occur postoperatively as well. In cases where the flow of saliva is inhibited, the static saliva acts as a nidus (a place where the development of something is promoted) for bacterial proliferation. Situations such as dehydration, trauma, or immunosuppression may predispose individuals to these conditions.
Patients with acute suppurative sialadenitis may have, in addition to painful swelling, cutaneous changes (erythema) and purulent (consisting of or containing pus) discharge from the ducts of the affected gland. Resident oral flora (Haemophilus influenza, Escherichia coli, Streptococcus pneumonia, and Staphylococcus aureus) are common offenders isolated from cultures of the purulent discharge.
Parotid gland surgery
Concerning that it is related to several important anatomical structures, the parotid gland has a special place in the world of surgery. The main goal in the surgical procedures that involve the parotid gland is to avoid injuring of the facial nerve, external carotid artery, auriculotemporal nerve and retromandibular vein. These structures can be easily remembered by using the mnemonic FEAR (Facial nerve, External carotid artery, Auriculotemporal nerve, Retromandibular vein).
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