A proposed fourth pair of salivary glands, thetubarial glands, were first identified in 2020. They are named for their location, being positioned in front of and over thetorus tubarius. However, this finding from one study is yet to be confirmed.[5]
The twoparotid glands are major salivary glands wrapped around themandibular ramus in humans.[6] These are largest of the salivary glands, secretingsaliva to facilitatemastication andswallowing, andamylase to begin the digestion ofstarches.[7] It is the serous type of gland which secretesalpha-amylase (also known asptyalin).[8] It enters theoral cavity via theparotid duct. The glands are located posterior to the mandibular ramus and anterior to themastoid process of thetemporal bone. They are clinically relevant in dissections offacial nerve branches while exposing the different lobes, since anyiatrogenic lesion will result in either loss of action or strength of muscles involved infacial expression.[8] They produce 25% of the total salivary content in the oral cavity.[7]Mumps is aviral infection, caused by infection in the parotid gland.[9]
The submandibular glands (previously known as submaxillary glands) are a pair of major salivary glands located beneath the lower jaws, superior to thedigastric muscles.[6] The secretion produced is a mixture of bothserous fluid andmucus, and enters theoral cavity via thesubmandibular duct or Wharton duct.[7] Around 70% of saliva in the oral cavity is produced by the submandibular glands, though they are much smaller than the parotid glands.[7] This gland can usually be felt via palpation of the neck, as it is in the superficial cervical region and feels like a rounded ball. It is located about two fingers above theAdam's apple (laryngeal prominence) and about two inches apart under the chin.
The sublingual glands are a pair of major salivary glands located inferior to the tongue, anterior to the submandibular glands.[6] The secretion produced is mainlymucous in nature, but it is categorized as a mixed gland.[8] Unlike the other two major glands, the ductal system of the sublingual glands does not have intercalated ducts and usually does not have striated ducts, either, so saliva exits directly from 8-20 excretory ducts known as theRivinus ducts.[8] About 5% of saliva entering the oral cavity comes from these glands.[7]
Thetubarial glands are suggested as a fourth pair of salivary glands situated posteriorly in thenasopharynx and nasal cavity, predominantly with mucous glands, and its ducts opening into the dorsolateral pharyngeal wall. The glands were unknown until September 2020, when they were discovered by a group of Dutch scientists usingprostate-specific membrane antigenPET-CT. This discovery may explainmouth dryness after radiotherapy despite the avoidance of the three major glands. However, these findings from just one study need to be confirmed.[10][5] On the other hand, an interdisciplinary group of scientists disagree with this new discovery. They believe that an accumulation of minor salivary glands has been described.[11] The researchers examined the scans and confirmed the glandular tissue through dissection of cadavers, proposing that this structure could constitute a previously unrecognized major salivary gland.[12] Their findings raised clinical interest, particularly in radiation oncology, as damage to these glands during radiotherapy for head and neck cancers might contribute toxerostomia (dry mouth), even when the known major salivary glands are spared.
Around 800 to 1,000 minor salivary glands are located throughout the oral cavity within thesubmucosa[13] of theoral mucosa in the tissue of the buccal, labial, and lingual mucosa, the soft palate, the lateral parts of the hard palate, and the floor of the mouth or between muscle fibers of the tongue.[14] They are 1 to 2 mm in diameter and unlike the major glands, they are not encapsulated by connective tissue, only surrounded by it. The gland has usually a number ofacini connected in a tiny lobule. A minor salivary gland may have a common excretory duct with another gland, or may have its own excretory duct. Their secretion is mainly mucous in nature and have many functions such as coating the oral cavity with saliva. Problems with dentures are sometimes associated with minor salivary glands if dry mouth is present.[13] The minor salivary glands are innervated by thefacial nerve (cranial nerve CN VII).[14]
Von Ebner's glands are found in a trough circling thecircumvallate papillae on the dorsal surface of the tongue near theterminal sulcus. They secrete a purely serous fluid that beginslipidhydrolysis. They also facilitate the perception oftaste through secretion of digestive enzymes and proteins.[13]The arrangement of these glands around the circumvallate papillae provides a continuous flow of fluid over the great number of taste buds lining the sides of the papillae, and is important for dissolving the food particles to be tasted.
Parasympathetic innervation to the salivary glands is carried viacranial nerves. The parotid gland receives its parasympathetic input from theglossopharyngeal nerve (CN IX) via theotic ganglion,[15] while the submandibular and sublingual glands receive their parasympathetic input from thefacial nerve (CN VII) via thesubmandibular ganglion.[16] These nerves release acetylcholine and substance P, which activate the IP3 and DAG pathways respectively.
Direct sympathetic innervation of the salivary glands takes place via preganglionic nerves in the thoracic segments T1-T3 which synapse in thesuperior cervical ganglion with postganglionic neurons that release norepinephrine, which is then received by β1-adrenergic receptors on the acinar and ductal cells of the salivary glands, leading to an increase in cyclic adenosine monophosphate (cAMP) levels and the corresponding increase of saliva secretion. Note that in this regard both parasympathetic and sympathetic stimuli result in an increase in salivary gland secretions,[17] the difference lies on the composition of this saliva, once sympathetic stimulus results particularly in the increase of amylase secretion, which is produced by serous glands. The sympathetic nervous system also affects salivary gland secretions indirectly by innervating theblood vessels that supply the glands, resulting in vasoconstriction through the activation of α1 adrenergic receptors, lessening the saliva's water content.
Secretory cells are found in a group, oracinus. Each acinus is located at the terminal part of the gland connected to the ductal system, with many acini within each lobule of the gland. Each acinus consists of a single layer of cuboidal epithelial cells surrounding a lumen, a central opening where the saliva is deposited after being produced by the secretory cells. The three forms of acini are classified in terms of the type of epithelial cell present and the secretory product being produced - serous, mucoserous, and mucous.[18][19]
In the duct system, the lumina are formed byintercalated ducts, which in turn join to formstriated ducts. These drain into ducts situated between the lobes of the gland (calledinterlobular ducts or secretory ducts). These are found on most major and minor glands (exception may be the sublingual gland).[18]
All of the human salivary glands terminate in the mouth, where the saliva proceeds to aid in digestion. The released saliva is quickly inactivated in the stomach by the acid that is present, but saliva also contains enzymes that are actually activated by stomach acid.
About 20,000 protein-coding genes areexpressed in human cells and 60% of these genes are expressed in normal, adult salivary glands.[20][21] Less than 100 genes are more specifically expressed in the salivary gland. The salivary gland specific genes are mainly genes that encode for secreted proteins and compared to other organs in the human body; the salivary gland has the highest fraction of secreted genes. The heterogeneous family of proline-rich, human salivary glycoproteins, such asPRB1 andPRH1, are salivary gland-specific proteins with highest level of expression. Examples of other specifically expressed proteins include the digestive amylase enzymeAMY1A, the mucinMUC7 andstatherin, all of major importance for specific characteristics of saliva.
This sectionneeds expansion. You can help byadding to it.(January 2018)
Salivary glands secrete saliva, which has many benefits for the oral cavity and health in general. The knowledge of normal salivary flow rate (SFR) is extremely important when treating dental patients.[24] These benefits include:
Protection: Saliva consists of proteins (for example; mucins) that lubricate and protect both the soft and hard tissues of the oral cavity. Mucins are the principal organic constituents of mucus, the slimy viscoelastic material that coats all mucosal surfaces.[25]
Buffering: In general, the higher the saliva flow rate, the faster the clearance and the higher the buffer capacity, hence better protection from dental caries. Therefore, people with a slower rate of saliva secretion, combined with a low buffer capacity, have lessened salivary protection against microbes.[26]
Pellicle formation: Saliva forms a pellicle on the surface of the tooth to prevent wearing. The film contains mucins and proline-rich glycoprotein from the saliva.
The proteins (statherin and proline-rich proteins) within the salivary pellicle inhibit demineralization and promote remineralization by attracting calcium ions.[27]
Maintenance of tooth integrity: Demineralization occurs when enamel disintegrates due to the presence of acid. When this occurs, the buffering capacity effect of saliva (increases saliva flow rate) inhibits demineralization. Saliva can then begin to promote the remineralization of the tooth by strengthening the enamel with calcium and phosphate minerals.[28]
Antimicrobial action: Saliva can prevent microbial growth based on the elements it contains. For example, lactoferrin in saliva binds naturally with iron. Since iron is a major component of bacterial cell walls, removal of iron breaks down the cell wall, which in turn breaks down the bacterium. Antimicrobial peptides such as histatins inhibit the growth of Candida albicans andStreptococcus mutans. Salivary immunoglobulin A serves to aggregate oral bacteria such as S. mutans and prevent the formation of dental plaque.[29]
Tissue repair: Saliva can encourage soft-tissue repair by decreasing clotting time and increasing wound contraction.[30]
Digestion: Saliva contains amylase, which hydrolyses starch into glucose, maltose, and dextrin. As a result, saliva allows some digestion to occur before the food reaches the stomach.[31]
Taste:[32] Saliva acts as a solvent in which solid particles can dissolve and enter the taste buds through oral mucosa located on the tongue. These taste buds are found within foliate and circumvallate papillae, where minor salivary glands secrete saliva.[33]
Asialolithiasis is a salivary stone that may cause blockage of the ducts, most commonly the submandibular ducts, causing pain and swelling of the gland.[34]
Cancer treatments includingchemotherapy andradiation therapy may impair salivary flow.[38][35] Radiotherapy can cause permanent hyposalivation due to injury to the oral mucosa containing the salivary glands, resulting in xerostomia, whereaschemotherapy may cause only temporary salivary impairment.[38][35] Surgical removal of benign or malignant lesions may also impair function.[39]
The salivary glands of some species are modified to produce proteins; salivary amylase is found in many bird and mammal species (including humans, as noted above). Furthermore, thevenomglands ofvenomous snakes,Gila monsters, and someshrews, are actually modified salivary glands.[37] In other organisms such asinsects, salivary glands are often used to produce biologically important proteins such assilk or glues, whilstfly salivary glands containpolytene chromosomes that have been useful ingenetic research.[43]
^abcBialek EJ, Jakubowski W, Zajkowski P, Szopinski KT, Osmolski A (2006). "US of the major salivary glands: anatomy and spatial relationships, pathologic conditions, and pitfalls".Radiographics.26 (3):745–63.doi:10.1148/rg.263055024.PMID16702452.
^abcdeNanci A (2018).Ten Cate's Oral Histology: Development, Structure, and Function (ninth ed.). Elsevier.ISBN978-0-323-48524-1.
^Valstar, Matthijs H.; de Bakker, Bernadette S.; Steenbakkers, Roel J. H. M.; de Jong, Kees H.; Smit, Laura A.; Klein Nulent, Thomas J. W.; van Es, Robert J. J.; Hofland, Ingrid; de Keizer, Bart; Jasperse, Bas; Balm, Alfons J. M.; van der Schaaf, Arjen; Langendijk, Johannes A.; Smeele, Ludi E.; Vogel, Wouter V. (2020-09-22)."The tubarial salivary glands: A potential new organ at risk for radiotherapy"(PDF).Radiotherapy and Oncology.154:292–298.doi:10.1016/j.radonc.2020.09.034.PMID32976871.
^abcNanci A (2013).Ten Cate's Oral Histology: Development, Structure, and Function (8th ed.). Elsevier. pp. 275–65.ISBN978-0-323-07846-7.
^abHerring MJ, Fehrenbach SW (2012).Illustrated Anatomy of the Head and Neck (4th ed.). Elsevier/Saunders.ISBN978-1-4377-2419-6.
^Frommer J (1977). "The human accessory parotid gland: its incidence, nature, and significance".Oral Surgery, Oral Medicine, and Oral Pathology.43 (5):671–6.doi:10.1016/0030-4220(77)90049-4.PMID266146.
^Ishizuka K, Oskutyte D, Satoh Y, Murakami T (2010). "Multi-source inputs converge on the superior salivatory nucleus neurons in anaesthetized rats".Autonomic Neuroscience: Basic & Clinical.156 (1–2):104–10.doi:10.1016/j.autneu.2010.03.014.PMID20435522.S2CID25907120.
^Uhlén M, Fagerberg L, Hallström BM, Lindskog C, Oksvold P, Mardinoglu A, et al. (January 2015). "Proteomics. Tissue-based map of the human proteome".Science.347 (6220) 1260419.doi:10.1126/science.1260419.PMID25613900.S2CID802377.
^Tabak LA, Levine MJ, Mandel ID, Ellison SA (February 1982). "Role of salivary mucins in the protection of the oral cavity".Journal of Oral Pathology and Medicine.11 (1):1–17.doi:10.1111/j.1600-0714.1982.tb00138.x.PMID6801238.
^Comba, Allegra."Saliva".flipper e nuvola. Retrieved25 February 2018.
![[icon]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg)
