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Salivary glands

The salivary glands (Latin: glandulae salivariae) are the digestive system's exocrine glands located in the oral cavity or close to it with openings in the mouth for excretion of produced secretion

Salivary glands produce and excrete a watery substance called saliva, contributing the digestion. Glands classify regarding their size and duct types. There are two groups of salivary glands- minor and major glands. 

Major glands are the primary glands providing the oral cavity and its structure moistening, lubrication, and protection. Gland-produced saliva starts the chemical food procession, and it helps in digestion, food lubrication during swallowing, and bolus creation. 

Minor salivary glands

Minor salivary glands locate in the oral cavity mucosa, and each of them excretes their secretion on the mucosa's surface

Minor glands mainly function for the oral cavity lubrication. They provide only around 1% of total daily saliva production. 

In total, the oral cavity contains about 800-1000 dispersed minor salivary glands. Glands divide regarding location. 

There are four groups of minor salivary glands in the oral cavity:

  • salivary glands in the lips (labial),
  • cheek salivary glands (buccal),
  • glands in the palate (palatine), dividing into hard and soft palate glands,
  • glands in the tongue (lingual).

The oral cavity inferior wall mucosa also contains minor salivary glands. Cheek and lip salivary glands open in the oral vestibule part, but the palate and tongue glands- in the oral cavity proper.

Major salivary glands

Major salivary glands are more considerable than minor ones. They are a group or a collection of exocrine tissue providing the saliva secretion through one central duct, which opens in the oral cavity's mucosa. 

Major gland-produced saliva mainly provides the initiation of digestion and protection.

There are three pairs of salivary glands- the parotid, submandibular, and sublingual salivary glands. 

Parotid salivary gland anatomy

The parotid salivary gland is the largest salivary gland. It weighs around 20-30 grams (0.71- 1.06 oz) and helps form the cheeks by giving fullness. It has an inverted pyramidal shape, and it produces around 60-65% of total saliva

The parotid gland has a fibrous capsule called the parotid sheath made by parotid fascia covering the gland from all sides.

Borders and location

It locates on the lateral face side, anterior to the outer ear's auricle part. The region is known as the parotid, also the preauricular area. Anterior to the parotid gland is the masseter muscleposterior- the external ear, and sternocleidomastoid muscleSuperior is the zygomatic arch, and inferior- the mandible and mandible's angle.

Parts

It has two parts- the superficial and deep part divided by the facial nerve. Between both is fatty tissue.

Superficial lobe

The superficial lobe locates lateral to the facial nerve. It lies subcutaneously in the preauricular area, anterior to the external auditory meatus

The gland lies next to the masseter muscle's outer surface. 

Superior it goes until the zygomatic archanterior till the masseter muscle's middle one-third, and inferior extends until the mandible angle

In the posterior direction, the superficial part turns into the deep part.

Deep lobe

The deep part locates in the retromandibular fossa and lies medial to the facial nerve

Its anterior part borders with mandible rams' posterior edge, but posterior with the temporal bone's mastoid process and sternocleidomastoid muscle. 

The deep part has a medial border with the temporal bone's styloid process and three muscles originating from it (stylohyoid, stylopharyngeus, and styloglossus).

Accessory parotid gland

In about 50% of individuals, the parotid gland has accessory minor parotid gland tissues located anterior and more in the parotid gland's superior part. 

Its inferior border connects with the parotid duct's superior edge. 

The accessory gland can have one or several smaller ducts connecting with the parotid duct.

Parotid duct and opening

Gland's duct or the parotid duct goes through the cheeks. The duct is also known as the Stensen's duct. It is around 5-7 cm (1.97 -2.76 inches) long and 1.5- 2.5 millimetres wide (0.06 -0.1 inches). 

The duct from the gland's anterior edge goes further below the zygomatic arch and wraps around the anterior margin of the master muscle. It passes horizontally over its surface. Next, it pierces through the buccinator muscle. 

In the oral cavity, the buccal mucosa opposite the second maxillar molar tooth presents the parotid duct's papilla. The duct opens through it and secrets saliva, helping in the digestion process and moistening the oral cavity.

Anatomical relations

Besides facial nerve passing through the parotid gland and dividing it, two more anatomical structures cross it- the external carotid artery and the retromandibular vein.

The facial nerve divides the gland into two parts and gives five terminal branches within the gland. 

More from glands superior position arises the temporal and zygomatic branches, while more from the central region- buccal, margin mandibular, and cervical branches.

The external carotid artery ascends through the gland, and its superior part gives two terminal branches- the superficial temporal and maxillary artery. The posterior auricular artery arises from it within the gland.

Maxillary and superficial temporal veins form the retromandibular vein within the gland and going through it.

Auriculotemporal branches from mandibular branch of the trigeminal nerve (CN V3) also crosses the gland.

Submandibular salivary gland anatomy

The submandibular salivary gland is the second largest from major salivary glands, and it locates below the oral cavity inferior wall muscles. Each gland weighs around 10 grams (0.35 oz), is elongated, a bit flattened and has the shape of the letter "U" or the horseshoe shape. 

Without the parotid gland stimulation, the submandibular gland can produce around 70% of the saliva. When the parotid gland receives a stimulus, the submandibular gland produces about 10-30% of saliva

The cervical fascia's superficial layer wraps around the gland and forms closed space. 

Parts, borders, and location

The submandibular gland has two parts- the superficial and deep

Both are continuous with each other, and the gland divides mylohyoid muscle. Its posterior border fibers extend in the gland, and it wraps around them. 

Gland's superficial part locates in the submandibular triangleThe deep portion lies in the submandibular fossa located in the mandible body's inner surface. 

The submandibular fossa locates on the lateral side of the mandible's mental spine, inferior to the middle portion of the mylohyoid line. 

The submandibular triangle is a part of the anterior neck triangle, and it borders with the mandible's body and anterior and posterior belly of the digastric muscle.

The superficial part is the most significant portion, locate more inferior. The deep part finds more medial.

The deep part participates in the oral cavity inferior wall formation and ends close to the sublingual gland's posterior surface. It locates between the mandible's body on one side and the hyoglossus muscle's lateral surface on the other.

The lingual nerve goes along the mandible facing the deep part's surface. It begins lateral to the submandibular duct and goes beneath it in the space between the sublingual and submandibular glands. And finally, it terminates by giving branches.

Submandibular duct and opening

Gland's saliva travels to the oral cavity by the submandibular duct. It is also known as the Wharton's duct. It connects the submandibular gland with the oral cavity proper and is around 4-5 cm (1.57-1.97 inches) long and 1.5-2 millimeters (0.06-0.08 inches) wide.

The submandibular duct wraps around the mylohyoid muscle's posterior edge and gets above the oral cavity inferior walls muscles. Next, it goes in the medial direction, and along the muscle's upper surface, it reaches the sublingual caruncle or papilla and opens through it. On the way to the caruncle, it crosses with the lingual nerve. 

Its mucosa forms a lingual frenulum connecting the tongue with the oral cavity floor in the midline of the tongue's ventral surface. The lingual frenulum ends with the sublingual caruncle, a minor swelling.

Sublingual salivary gland anatomy

Sublingual salivary glands are the smallest of the major salivary glands, and they weigh only around 5 grams (0.18 oz). Glands contribute about 5% of total saliva in the oral cavity. They have a longitudinal direction and lentiform shape when viewing along the mandible's body, a little bit resembling almonds in form.

Borders and location

Glands locate above the oral cavity inferior wall muscles. Mouth's floor mucosa covers them. Above the mucosa lies the tongue's lateral aspects. 

Gland's inferior part lies next to the superior surface of the mylohyoid muscle. In the medial direction, it borders with the genioglossus and hyoglossus muscles

On the medial side along the gland goes the lingual nerve and submandibular duct. Its lateral and anterior part faces the mandible and lies in the sublingual fossa located in the mandible's body part's inner surface. 

Both sublingual glands have a close connection in the mandible's mental spine area.

Sublingual fossa locate on the lateral side of the mandible's mental spine, superior to the medial end of the mylohyoid line. 

Sublingual duct and opening

The gland covers the oral cavity's mucosa forming the sublingual folds or plicas in the floor part. Plicas go in the lateral direction from the sublingual caruncle. 

The sublingual gland classifies as a major salivary gland. However, it has one major and several minor ducts secreting saliva directly into the oral cavity. This mechanism usually contains minor salivary glands. 

The central duct is also called the Bartholin duct, but the minor ducts are known as the Rvinus ducts

The Bartholin duct near the sublingual caruncle joins the submandibular duct and, together with it, opens in the sublingual caruncle. On sublingual plicas along the sides and towards the caruncle part opens the sublingual salivary gland's minor ducts. 

Microanatomy

All major glands are exocrine-type glands. The surface on which saliva excretes they reach through the ducts. Minor glands are compound glands with a simple duct system, while major salivary glands are compound, containing ducts with several branches.

All major glands have stroma and parenchyma parts

The stroma is the gland's connective tissue part, but parenchyma consists of secretory cells. 

Stroma and duct system

All major glands cover the connective tissue capsule from which tissue goes into the glands, dividing them into smaller lobules

The parotid gland has not only lobules but also lobes as it is more significant than other glands. The parotid gland's capsule's tissue divides the gland into lobes, and connective tissue from the lobes divides them into lobules.

Between the lobules are the parenchyma consisting of secretory cellsLobules contain two different intralobular ducts- the intercalated and striated ducts. Striated ducts arise from the intercalated. 

The intercalated ducts are the smallest ones, and they drain individual acini. Outside the lobules are extra lobular or interlobular ducts that form the central excretory duct. 

The central duct's epithelium usually is simple columnar, pseudostratified columnar, or stratified cuboidal

Secretory units

The secretory cell unit of the salivary glands is called the acini. They contain various cell organizations regarding the content of the product they release. All glands have many myoepithelial cells helping in the excretion process by emptying the ducts from saliva.

Salivary glands' produced saliva can be serous or mucous

Mucous cells are rich in carbohydrates and mucinswhile serous- rich in proteins

The acinar organization has serous cells but tubular- mucous cells.

Minor salivary glands mostly have the mucous type and tubular units.

Parotid gland

The parotid gland is a compound acinar gland, and it is the only serous gland. The parotid gland's intralobular connective tissue contains many blood vessels and fat cells, increasing with aging.

Submandibular gland

Submandibular is a mixed gland, meaning it contains serous and mucous secretory units. It is a compound acinotubular typeMost of the acini are mixed or only serous, so it mainly produces serous saliva. Serous cells make up about 90-95%, while mucous only around 5-10%. The submandibular gland is the only gland functioning as an exocrine and an endocrine gland. Its striated ducts produce insulin-like substances and nerve growth factors.

Sublingual gland

Sublingual is also a compound acinotubular gland. Gland's acini are mixed or mucous, so the sublingual gland mainly produces mucous saliva, although it can also produce serous saliva. 

Functions

All salivary gland's primary function is the production of saliva, regardless of the size of glands. 

Saliva is a seromucous transparent and stretchy liquid with a pH of 6.2-7.6 and several essential functions. It mainly contains water- around 99.5%. All salivary glands together produce about 1-1,5 liters of saliva per day.

  • Saliva regulates the balance of acids and alkalines in the oral cavity by buffering pH. When an individual ingests very acidic food, saliva neutralizes the acid and protects the teeth' enamel.
  • Saliva moistens and lubricates the mucosa and food. It protects the mucosa from mechanical forces and helps form the bolus during chewing. Food lubrication allows for swallowing the food more efficiently and aids in digestion.
  • Lubrication also has a protective role because it inhibits or makes it harder for bacteria to attach the epithelium.
  • Saliva starts the chemical food procession in the oral cavity as it contains an enzyme called amylase that splits carbohydrates.
  • Saliva contains proteins, mucus, electrolytes, and inorganic substances such as minerals. Saliva supplies the teeth with minerals (for example, it contains calcium salts).
  • They promote the action of taste buds as they dissolve water-soluble nutrients.
  • Saliva helps in food temperature regulation making the hot food easy to digest by lowering its temperature when mixed with saliva.
  • Saliva provides dental hygiene and dental protection by helping to wash away food remains after digestion.
  • It helps in antimicrobial protection. Saliva contains substances such as lysozyme, IgA, and secretory protein complexes assisting in the process.
  • Healthcare professionals use saliva in disease detection and diagnosis. Saliva releases mercury and iodine in case of poisoning.
  • During embryological development, saliva stimulates the growth of taste buds and odontoblast differentiation.
  • Saliva helps maintain a constant pH and normal oral microflora.

Neurovascular supply

Arterial blood supply

The parotid gland's arterial blood supply provides maxillary artery branches, posterior auricular, superficial temporal, and transverse facial artery.

Submandibular salivary gland perfuses submental artery and glandular branches from facial artery.

The sublingual gland gets the supply from sublingual and submental arteries. The first is the lingual artery branch, but the second- facial.

Facial and lingual arteries are external carotid artery branches, and the superficial temporal and maxillary arteries are its terminal branches.

Venous drainage

The parotid gland veins drain to the superficial temporal vein, next to the retromandibular vein, and then next to the external jugular vein. 

The pterygoid plexus drains the deep part, carrying the blood next to the maxillary vein and then to the retromandibular. Maxillary and superficial temporal veins form the retromandibular vein. 

The facial vein drains the superficial part carrying the blood next to the internal jugular vein.

The submandibular glands venous drainage provides the facial and sublingual vein carrying blood to the internal jugular vein. The sublingual vein drains into the lingual vein.

The sublingual glands drain to the sublingual and submental vein, draining next to the lingual vein and facial veins carrying the blood to the internal jugular vein.

Lymphatic drainage

Lymphatic drainage of the parotid gland provides the superficial and deep cervical lymph nodes via local nodes such as infraparotid and paraparotid nodes. The parotid gland surrounds many regional lymph nodes creating a lymphatic network providing the lymphatic drainage for surrounding tissue and structures such as the auricle or middle ear's structures. Most of all nodes, they locate close to the superficial parotid gland's part.

Submandibular and sublingual glands drain to submandibular and deep cervical lymph nodes.

Innervation

All salivary glands supply the autonomic nerve system's both parts- the sympathetic and parasympathetic.

Both parts regulate salivary secretion. Increased parasympathetic stimulation leads to increased and stimulated saliva production. But the sympathetic system decreases and inhibits the secretion by reducing the blood flow through vasoconstriction. It leads to more mucus saliva.

The parotid gland receives autonomic and sensory innervation. 

The gland gets the parasympathetic innervation from the preganglionic lesser petrosal nerve fibers. Fibers are from the glossopharyngeal nerve (CN IX). 

CN IX goes to the otic ganglion. The parotid gland receives postganglionic secretomotor otic ganglion fibers by the auriculotemporal nerve. It is a branch of trigeminal nerve’s mandibular branch (CN V3). 

The sympathetic innervation provides the superior cervical ganglion. 

The parotid gland also innervates the auriculotemporal nerve's sensory fibers (the mandibular nerve branch (CN V3)). But great auricular nerve from the cervical plexus innervates the gland's fascia.

The submandibular and sublingual glands get parasympathetic innervation from the preganglionic chorda tympani fibers.

Fibers come from the intermediate nerve, a part of the facial nerve (CN VII). They go to the submandibular ganglion. 

Both glands receive the innervation from their postganglionic parasympathetic secretomotor glandular fibers. 

The sympathetic innervation glands get from the superior cervical ganglion fibers.

Both glands get the sensory innervation by the lingual nerve from the mandibular nerve, a trigeminal nerve branch (CN V3).

Salivary gland disorders

Salivary glands have an essential role in digestion initiation. 

If the gland is damaged, it can lead to decreased saliva production, causing problems with digestion, food swallowing and chewing, infection development in the oral cavity, teeth disorders, and many more issues to follow. 

Salivary gland disorders present as an isolated disease, affecting only the gland itself. Or it can be involved in manifestations of systemic diseases such as Sjogren's syndrome- an autoimmune disorder affecting organs producing secretion (one of the symptoms is mouth dryness and salivary gland swelling because of decreased saliva production).

The most common causes for salivary gland disorders are:

  • duct obstruction by stones or duct twists,
  • infections and inflammation
  • systemic and autoimmune diseases, 
  • tumors
  • a large amount of alcohol consumption can lead to swollen glands. 

Disorder prevention includes:

  • drinking plenty of water, at least 1.5 liters (50.7 oz) per day, 
  • eating regularly and healthy, 
  • avoiding food that can stick to the mouth's roof, 
  • good oral hygiene practicing,
  • teeth cleaning and mouth rinsing after meals, 
  • Smoking avoidance.

Sialolithiasis

Sialolithiasis is a disorder characterized by stone formation in salivary gland ducts

Various factors causing decreased saliva secretion can cause calcium salt precipitation resulting in calcium-rich stone formation. Most often, the condition presents in males between the age of 30-65 years

Sialolithiasis is the most common cause of salivary gland swellingMost often, the disorder affects the submandibular salivary gland and its ducts. It's because the central duct ascends, and saliva drains towards the sublingual caruncle. The submandibular gland is a mixed gland, and it produces serous and mucous saliva that is more viscous, resulting in a slower flow.

Sublingual and minor glands rarely have stones. 

While the exact mechanism is unknown, there are several factors related to stone formation, such as 

  • decreased water intake or dehydration as it leads to thickened saliva, 
  • reduced food intake as it lowers the saliva production, 
  • certain medications also minimize saliva production (some antidepressants, antihistamine, blood pressure medications, diuretics), 
  • systemic disease's (Sjogren's syndrome, systemic lupus erythematosus) 
  • mouth radiation therapy
  • smoking
  • gout.

Stone can be present without any symptoms, but symptoms are present if stuck in one of the ducts. The severity of symptoms depends on duct blockage- entirely or just partially blocked.

It presents with a painful involved gland's swelling and pain worsening by eating. Usually few hours after eating, the pain disappears as the saliva flow normalizes. An individual can have difficulties opening the mouth and difficulty swallowing, and mouth dryness can be present.

Small stones usually pass out on their own, while larger stones typically need surgical removal.

Untreated sialolithiasis can lead to infection, inflammation, or even abscess formation.

Sialadenitis

Sialadenitis is salivary gland infection and inflammation. More often, elderly adults and people who already have sialolithiasis present with it. The condition can be acute or chronic

Most often, it affects the parotid gland, so it is called parotitis.

The condition can cause infections, salivary gland stones, autoimmune diseases (Sjogren's syndrome, Wegner's granulomatosis), or the cause can be unknown.

Acute sialadenitis usually cause infections and inflammations but chronic- autoimmune disorders

From infections, the most common cause is a viral infection called mumps. Other viral infections include HIV, parainfluenza, Epstein-Barr virus, cytomegalovirus, coxsackievirus. The most common bacteria causing sialadenitis is the Staphylococcus aureus.

Usually, this disorder presents as a painful and tender lump in the salivary gland area. An individual can have increased temperature and even fever, chills, and weakness. In more severe cases, purulent discharge from the duct's opening can present in the oral cavity.

The most proper treatment for this condition is hydration and analgesia therapy and treatment for the cause. For example, in case of bacterial infection- antibiotics should be given. Usually, after medication ingestion condition soon start to improve. 

Often lemon juice or hard candy is given to promote saliva production.

Severe cases can result in abscess formation, but most patients have a good prognosis, and the condition is self-limiting or are easy to treat.

Mumps

Mumps has a viral etiology, and it causes mumps virus from the Paramyxoviridae family. Usually, mumps affects the parotid gland, and the disorder is known as mumps parotitis, causing swelling in both or only one gland. 

Swelling can also present with pain on the gland location site or during chewingHeadaches, fever, and weakness can be present, as well as appetite loss and muscle aches.

The virus is very contagious, and it is spread through direct contact or in a respiratory way. Before infection presents with parotid salivary gland swelling, it can manifest with headache, fatigue, low-grade pyrexia.

Mump-caused parotitis can lead to potentially dangerous complications such as:

  • orchitis, 
  • hearing loss, 
  • meningitis and encephalitis, 
  • pancreatitis.

 In pregnant women, the infection can cause miscarriage. 

There is no specific treatment. The best possible prevention is the MMR vaccination.

Summary on salivary glands

What are the three salivary glands?

The human organism has three pairs of major salivary glands- the parotid, submandibular and sublingual.

Where are the three salivary glands located?

The head and neck have three salivary gland pairs. The parotid gland locates anterior to the outer ear, the submandibular gland is located below the oral cavity's inferior wall's muscles, while the sublingual is above them.

Which is the largest salivary gland?

The largest salivary gland is the parotid salivary gland.

Which is the smallest salivary gland?

The smallest salivary gland from major salivary glands is the sublingual gland, but lips, tongue, hard and soft palate, cheeks contain minor salivary glands.

What does the parotid gland do?

The primary function of the parotid gland is saliva excretion.

How big are submandibular glands?

The submandibular gland has the shape of the flattened letter "U" or the horseshoe-like form, and it weighs around 10 grams (0.35 oz).

Are there two submandibular glands?

Yes, there are. Each side of the face contains one submandibular gland.

Where is the submandibular gland located?

The submandibular gland lies in the submandibular triangle. 

It also lies in the submandibular fossa located in the mandible body's inner surface. It finds below the oral cavity's inferior wall's muscles.

What do submandibular glands do?

The submandibular gland's primary function is saliva secretion.

What is a sublingual gland?

The sublingual gland is the smallest major salivary gland producing saliva and located under the oral cavity floor's mucosa.

Where are the sublingual glands located?

Sublingual glands locate above the oral cavity's inferior wall's muscles. It finds under the oral cavity's floor's mucosa.

What causes swollen salivary glands?

Swollen salivary glands can cause inflammation, infection, systemic autoimmune diseases, certain medications, dehydration, tumors, alcohol consumption, duct obstruction, and many more to follow.

What viral infection causes swollen salivary glands?

Swollen salivary glands can cause mumps, HIV, parainfluenza, Epstein-Barr virus, cytomegalovirus, coxsackievirus.

Can dehydration cause parotid gland swelling?

Yes, dehydration can cause parotid gland swelling.

Which salivary gland is affected by the mumps?

All salivary glands can affect mumps, but most commonly virus affects parotid glands, sometimes submandibular glands, and very rarely sublingual glands.

What are the symptoms of parotitis?

Parotitis presents with a painful and tender lump in the parotid gland area. Usually, pain is in the location site and during chewing. The skin can have redness. An individual can have increased temperature and even fever, chills and weakness, appetite loss, and muscle aches. In more severe cases, purulent discharge from the duct's opening can present in the oral cavity.

What is sialolithiasis?

Sialolithiasis is a disorder characterized by stone formation in salivary gland ducts. 

What causes a clogged salivary gland?

Clogged or obstructed salivary gland most often causes salivary gland stones.

What are the symptoms of a blocked salivary gland?

The blocked salivary gland can present with a painful lump and involved gland's swelling, and pain worsening by eating. Usually few hours after eating, the pain disappears as the saliva flow normalizes. An individual can have difficulties opening the mouth and difficulty swallowing. Mouth dryness can be present.