Salivary glands are a very important exocrine gland in the body. Three major paired salivary glands produce the majority of the saliva i.e. sub mandibular gland, sublingual gland and Parotid gland. In addition to it about 600 to 1000 minor salivary gland line the oral cavity and Or pharynx which contribute towards the saliva in total.
The major salivary glands develop during the 6th week of intra uterine life from the outpouchings of oral ectoderm into the surrounding mesenchyme. The parotid gland is the first to develop, growing in the posterior direction as the facial nerve advances anteriorly. However it is also the last to be encapsulated after the formation of lymphatic supply of the gland. That is why sometimes there is entrapment of lymphatics in the parenchyma of the gland. This is important as can be a major cause of formation of Warthin's tumor and Lymph epithelial cyst. All the other salivary glands don't have inter parenchymal lymph nodes. The minor salivary glands develop from oral ectoderm and nasopharyngeal ectoderm.
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The parotid gland is the largest salivary gland of the body with an average weight of 14.28gm. It is wedge shaped and unilobular in structure. It is about 3.4 cm in width and 5.8 cm in height. It has 5 processes Out of which 3 are super facial and 2 are deep. It lies in a parotid compartment, a triangular which contains 7 cranial nerve and its branches, sensory and autonomic nerves, the external carotid arteries and its branches, the retromandibular vein and parotid lymphatics. The boundaries of the Parotid compartment are:
The mandible and the Masseter are overlapped by the gland at almost around 80 percent; the remaining 20 percent extends medially through the stylomandibular tunnel formed by the posterior edge of the mandibular ramus, SCM and posterior belly of the digastrics and stylomandibular ligament. The isthmus of the gland runs between the mandibular ramus and the post belly of the digastric to connect the retromandibular portion to the remainder of the gland. The parapharyngeal space is an inverted pyramid having base at the skull base while its apex is present at the greater cornu of the hyoid bone and is bounded medially by the pharyngeal wall and the mandibular ramus at lateral end and medially by pterygoid. This space is further styloid divided into pre- and post- styloid compartments with line joining the styloid process and medial pterygoid plate. The tail of the parotid overlies the upper Â¼th of the Sternocleidomastoid muscle and extends toward the mastoid process. Stensen's duct (parotid duct) arises from the anterior border of the Parotid and parallels the Zygomatic arch, 1.5 cm (approximately 1 finger breadth) inferior to the inferior margin of the arch. Stensen's duct runs superficial to the masseter muscle, and then turns medially 90 degrees to pierce the Buccinator muscle at the level of the second maxillary molar where it opens onto the oral cavity.
The submandibular gland is almost half the weight of the parotid gland weighing about 10 to15 grams. It occupies most of the submandibular triangle of the neck, the gland is actually folded around the free edge of mylohyoid muscle. There are no separate lobes to the glands still it is referred to be divided into superficial and deep lobes. The boundries of the gland are
Anteriorly & Inferiorly: Anterior belly of Digastric muscle
Posteriorly: Posterior belly of Digastric Muscle & Stylohyoid Ms.
Laterally: Lower border of Mandible
Medially: Medial pterygoid Ms.
Floor: The floor is formed by Mylohyoid & Hyoglossus Ms
The mandibular branch of CN VII passes superficial to the submandibular gland and deep to the platysma.Â The Submandibular gland has its own capsule which move with the superficial layer of deep cervical fascia.Â
The Wharton's duct of submandibular gland is present on the medial surface of the gland and extends between the mylohyoid and hyoglossus and on to the genioglossus muscle. It drains into the oral cavity lateral to the lingual frenulum on the anterior floor of mouth. The duct has average length of 5 cm. The Wharton's duct is surrounded by lingual nerve starting lateral and ending medial to the duct and CN XII parallel to it.
The diagram below shows that the submandibular gland is responsible for the production of nearly seventy percent of the saliva in the body. The Wharton duct which opens in the floor of the mouth develops in the ectoderm from a groove in the floor of the mouth.
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The lymph nodes in and around the gland drain into the submandibular gland.
Anatomical Relation of Submandibular Gland
This is the smallest of the major salivary glands. The shape of the gland is almond shape. It is just deep to the floor of the mouth mucosa between the mandible and genioglossus muscle. It is bounded inferiorly by the mylohoid muscle. Wharton's duct and the Lingual nerve pass between the sublingual gland and genioglossus muscle. The sublingual gland has no facial capsule like major salivary glands. It also lacks the dominant duct. Instead it is drained by 10 small ducts (The ducts of Rivinus), they open on the floor of the mouth. Several of the anterior ducts join together to form a common duct that is the Bartholin's Duct which opens into Wharton's duct.
Ref: Salivary glands anatomy and physiology; Peter M.Som & Margaret S.
Minor Salivary Glands:
The minor salivary glands are about 600 to 1000 in total lining the oral mucosa and the orophyranx. It lacks the branching network of the draining ducts. Each of the salivary unit has its own simple duct. The minor salivary glands are concentrated in the Buccal, Labial, Palatal and Lingual regions. They may also be found at the superior pole of tonsil (Weber's glands), the tonsillar pillars, and the base of tongue (Von ebner's glands), paranasal sinuses, larynx, trachea and bronchi.
Microanatomy of the Salivary Glands:
The secretory unit (salivary unit) consists of the acinus, myoepithelial cells, the intercalated duct, the striated duct, and the excretory duct. All salivary acinar cells contain secretory granules; in serous glands, these granules contain amylase, and in mucous glands, these granules contain mucin. Acini, responsible for producing the primary secretion, are divided into 3 types:
1) Serous (protein-secreting) spherical cells rich in zymogen granules
2) Mucous (mucin-secreting) more tubular shaped cells; mucinogen granules are washed out on H&E preparations giving an empty cell appearance
3) Mixed=serous demilunes, or predominantly mucous acinar cells capped by a few serous acinar cells
The Parotid gland is serous in nature and submandibular is mucous. The sublingual gland is mixed as it contains both serous and mucous cells in it.
Functions of Saliva:
At least 8 major functions of saliva have been identified
1) Moistens oral mucosa. In fact, the mucin layer on the oral mucosa is thought to be the most important nonimmune defense mechanism in the oral cavity.
2) Moistens dry food and cools hot food.
3) Provides a medium for dissolved foods to stimulate the taste buds.
4) Buffers oral cavity contents. Saliva has a high concentration of bicarbonate ions.
5) Digestion. Alpha-amylase, contained in saliva, breaks 1-4 glycoside bonds, while lingual lipase helps break down fats.
6) Controls bacterial flora of the oral cavity.
7) Mineralization of new teeth and repair of precarious enamel lesions. Saliva is high in calcium and phosphate.
8) Protects the teeth by forming a "Protective Pellicle". This signifies a saliva protein coat on the teeth which contains antibacterial compounds. Thus, problems with the salivary glands generally result in rampant dental caries
Fig: Functions of Saliva & its Contents
Ref: Salivary Proteins & Caries; Jelena Kosoric 2010
Role of Saliva in Remineralization:
Composition & Division of Saliva Production:
Saliva is composed predominantly of water (99.5 %) with a specific gravity of 1.002 to 1.012. Normally about 1 to 1.5 liters of saliva are produced daily, mostly during the meals. The parotids contribute about 45 % (450 to 675 ml) of the total secretions, the submandibular glands 45% (450 to 650ml) minor salivary glands 5 %. The basal secretion rate is about 0.04 ml/ min/ gland. During sleep this rate decreases almost to zero.
Saliva is formed as a result of flexes triggered by stimulation of taste buds and by activity of the muscles, joint and pressure produced on the teeth due to chewing. The major salivary glands control the production of the saliva by the autonomic nervous system. The saliva helps to maintain the environment of the oral cavity. The saliva maintains the environment of the oral cavity by the body's own natural protection mechanism to prevent tooth decay. It also protects soft tissue and lubricates oral cavity by mucins and glycoprotein. It has also anti-microbial action by breaking down bacterial cell walls and also inhibits growth of bacteria. The bicarbonate ions in saliva provide buffer by neutralizing acid production and also maintain plaque pH. The calcium and phosphates play a significant role in repair the early tooth decay. The dental caries is due to an imbalance between demineralization and remineralization.
Critical PH :
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Saliva is supersaturated with ions forming mineral content of the teeth which are the calcium, phosphate and hydroxyl ions. The critical value of PH is about 5.5.The saliva and plaques are unsaturated below this value and as a result tooth dissolves. If the value is above the calcium and phosphate ions from saliva initiate the repair process of the damaged mineral crystals of enamel. The PH at which there is neither demineralization nor remineralization taking place with in the oral environment is known as the critical PH.
Calcium and Phosphate ion System:
During unsaturation the hydroxyapatite crystals of the tooth mineral dissolve to discharge calcium, phosphate and hydroxyl ions. Below the critical pH, the increased concentration of hydrogen ions reacts with the hydroxyl ions to form water and with the phosphate ions to form hydrogen phosphate. This results in lowering the saturation and cause demineralization. If above saturation, the reaction will repair the damaged crystals by the entering of ions from solution.
The plaque pH decreases as a result of acid produced by the bacteria due to fermentation of carbohydrates mainly sugars from foods and drinks. After five to ten minutes of intake of food and drinks the acid drop the pH to a level low and the minerals from the tooth's enamel is dissolved (demineralization) resulting in white spot lesions which can lead to dental caries.
The plaque pH is increased by washing and neutralizing action of saliva, which has an important buffer, bicarbonate ion to stop demineralization. The calcium and phosphate enters from saliva and start repairing damaged mineral crystals from the enamel (remineralization). The pH also rises by the production of alkali such as ammonia, from nitrogenous compounds may present in foods and saliva.
Fluoride and Magnesium ion System:
Fluoride plays an important role in the remineralization process in the oral environment. It can replace the hydroxyl ion in the crystal structure of the enamel and can form a strong lattice system. The fluoride also acts as an anti carious factor. It forms flouroapitite crystal instead of normal crystal of hydroxyapitite. The flouroapitite crystal is more stable structure showing more resistance to the effect of acids and demineralization. It is less porous as well. According to one estimate about 27 percent of the enamel structure can replace hydroxyl group with the fluoride. Similarly the Magnesium can also form the crystal lattice in the oral cavity through the interaction of saliva.
Fig: Remineralization & Demineralization Ref: Salivary Proteins & Caries; Jelena Kosoric 2010
However the important and interesting thing to note is that fluoride ion can not pass the membrane or biofilm normally but in case of cariogenic surface the fluoride can easily pass through without any problem. That is why the surface around the caries in tooth is always rich in fluoride content. The fluoride ion when enters the membrane causes the decrease of an enzyme that is enolase which is required by the bacteria to metabolize carbohydrate and thus stopping indirectly the growth of the carious process.
Biofilm & Pellicle Formation:
The formation of plaque on the tooth surface as a microbial community is known as a biofilm. Saliva forms an organic pellicle on the tooth surface which is rich in glycoprotein. The bacteria are attached on the surface of the tooth by means if pellicle and forms microbial community. The flow of saliva provides nutrition to the biofilm along with removing the waste. Thus helping the tooth and oral environment in maintaining the homeostasis and prevent from repeated acid attacks. It also acts as a reservoir for the different ions, immuno proteins and antibodies like mucin, lysozyme, lactoferin, IgA, IgG which help against foreign organism and thus prevents aggregation as well.
Role of Proline Rich proteins like Histatim , Cystatins, Statherins etc in Saliva:
These complex proteins are present in the saliva and play an important role in clearing the bacteria from the surface of the tooth as well as preventing aggregation. They play a vital role in the remineralization by attaching themselves to the structure of calcium and phosphate as well as hydroxyapatite.
They bind to the crystalline structure and prevent the primary precipitation of the crystal as well as inhibiting the secondary crystallization (crystal growth). Of these proteins statherin attaches it self to the calcium structure and control the release of its ions in the oral environment as and when it is required. The low Ca ion in the saliva along with increase acid or decrease Ph causes demineralization while increase Ca ion and high Ph causes remineralization.