Salivary Glands and Saliva

Salivary Glands and Saliva

Oral cavity • Entrance door of the gastrointestinal tract; • Beginning of the digestive function; • Provides security of the body against food and drink; • Prior to the adoption act two strong evolutionary verified sensory systems: • Appearance; • Flavor; • When they enter in the mouth triggers the umbrella of the immune system: • Cell-associated protection: • Phagocytes and lymphocytes; • Secretory immunity system mainly protects mucous with secretoryIgA.

Additional protection in the mouth • Taste - taste buds; • Tactile sense - proprioception ; • Saliva.

Liquid oral environment • This is the liquid into the oral cavity, washing mucosa and dental enamel; • It consists of: • Saliva; • Gingival sulcus fluid; • Peeling epithelial cells and their degradation products; • Microorganisms and their products.

Saliva • Saliva is a complex fluid that in health almost continually bathes the parts of the tooth exposed within the oral cavity; • Consequently, saliva represents the immediate environment of the tooth.

Production • Saliva is produced by three paired sets of major salivary glands: • Parotid; • Submandibular; • Sublingual glands; • and by the many minor salivary glands scattered throughout the oral cavity.

Account of the composition • A precise account of the composition of saliva is difficult because not only are the secretions of each of the major and minor salivary glands is different, but their volume may vary at any given time. • In recognition of this variability, the term mixed saliva has been used to describe the fluid of the oral cavity.

Saliva has several functions • Saliva moistens the mouth; • Facilitates speech; • Lubricates food; • Helps with taste by acting as a solvent for food molecules; • Saliva also contains a digestive enzyme (amylase); • Saliva dilutes noxious material mistakenly taken into the mouth; • Cleanses the mouth; • Furthermore, it contains antibodies and antimicrobial substances; • Its buffering capacity plays an important role in maintaining the pH of the oral cavity.

There are three major pairs of salivary glands that differ in the type of secretion they produce: • parotid glands - produce a serous, watery secretion. • submaxillary (mandibular) glands - produce a mixed serous and mucous secretion. • sublingual glands - secrete a saliva that is predominantly mucous in character.

Three pairs of major salivary glands • They are located outside the oral cavity, with extended duct systems through which the gland secretions reach the mouth.

Numeroussmaller minor salivary glands • They are located in various parts of the oral cavity • the labial, • lingual, • palatal, buccal, • glossopalatine, • and retromolar glands— • They are typically located in the submucosallayer with short ducts opening directly onto the mucosal surface. - Minor mucous salivary gland, located in the submucosa below the epithelium of the oral cavity.

Composition of Saliva PARAMETER CHARACTERISTICS 600-1000 mL/day Na+, K+, Cl, HCO3−, Ca2+, Mg2+, HPO42−, and F– Amylase, proline-rich proteins, mucins,histatin, cystatin, peroxidase, lysozyme, lactoferrin, defensins, and cathelicidin-LL37 Secretory immunoglobulin A; immunoglobulins G and M; Glucose, amino acids, urea, uric acid, and lipid molecule Epidermal growth factor, insulin, cyclic adenosine monophosphate–binding proteins, and serum albumin • Volume • Electrolytes • Secretory proteins/Peptides • Immunoglobulins • Small organic • Other components

The basic secretory units of salivary glands are clusters of cells called an acini; • These cells secrete a fluid that contains water, electrolytes, mucus and enzymes, all of which flow out of the acinus into collecting ducts.

Acinar epithelial cells • Two basic types of acinar epithelial cells exist: • serous cells, which secrete a watery fluid, essentially devoid of mucus. • mucous cells, which produce a very mucus-rich secretion. • Small collecting ducts within salivary glands lead into larger ducts, eventually forming a single large duct that empties into the oral cavity.

Superficial temporal vessels Transverse facial artery Parotid duct Parotid gland Branches of facial nerve Facial artery Great auricular nerve Submandibular gland Anterior facial vein Masseter muscle Sternocleidomastoid muscle

The parotid gland is the largest salivary gland. • The superficial portion of the parotid gland is located subcutaneously, in front of the external ear, and its deeper portion lies behind the ramus of the mandible. • The parotid gland is associated intimately with peripheral branches of the facial nerve; • The parotid gland receives its blood supply from branches of the external carotid artery as they pass through the gland.

The duct (Stensen’s duct) of the parotid gland runs forward across the masseter muscle, turns inward at the anterior border of the masseter, and opens into the oral cavity at a papilla opposite the maxillary second molar.

Submandibular duct Tongue Sublingual ducts Sublingual gland Submandibular gland The major glands are bilaterally paired and have long ducts that convey their saliva to the oral cavity.

Submandibular gland • The submandibular gland is situated in the posterior part of the floor of the mouth, adjacent to the medial aspect of the mandible and wrapping around the posterior border of the mylohyoidmuscle; • The excretory duct (Wharton’s duct) of the submandibular gland runs forward above the mylohyoidmuscle and opens into the mouth beneath the tongue at the sublingual caruncle, lateral to the lingual frenum. • The submandibular gland receives its blood supply from the facial and lingual arteries. • The parasympathetic nerve supply is derived mainly from the facial nerve, reaching the gland through the lingual nerve and submandibularganglion. Submandibular gland

Sublingual gland • The sublingual gland is the smallest of the paired major salivary glands; • The gland is located in the anterior part of the floor of the mouth between the mucosa and the mylohyoid muscle; • The secretions of the sublingual gland enter the oral cavity through a series of small ducts (ducts of Rivinus) opening along the sublingual fold and often through a larger duct (Bartholin’s duct) that opens with the submandibular duct at the sublingual caruncle. • The sublingual gland receives its blood supply from the sublingual and submental arteries. • The facial nerve provides the parasympathetic innervation of the sublingual gland, also via the lingual nerve and submandibular duct at the sublingual caruncle Sublingual gland

The minor salivary glands • They are estimated to number between 600 and 1000, exist as small, discrete aggregates of secretory tissue present in the submucosa throughout most of the oral cavity. • The only places they are not found are the gingivaand the anterior part of the hard palate. • They are predominantly mucous glands, except for the lingual serous glands (Ebner’s glands) that are located in the tongue and open into the troughs surrounding the circumvallate papillae on the dorsum of the tongue and at the foliate papillae on the sides of the tongue.

Salivary gland showing its lobular organization. • Thicker partitions of connective tissue (septa), continuous with the capsule and within which run the nerves and blood vessels supplying the gland, invest the excretory ducts and divide the gland into lobes and lobules. Lobule Connective tissue septum

A salivary gland may be likened to a bunch of grapes. • Each “grape” is the acinus or terminal secretory unit, which is a mass of secretory cells surrounding a central space; • The spaces of the acini open into ducts running through the gland that are called successively the intercalated, striated, and excretory ducts analogous to the stalks and stems of a bunch of grapes. • These ducts are more than passive conduits, however their lining cells have a function in determining the final composition of saliva. • The ducts and acini constitute the parenchyma of the stroma carrying blood vessels and nerves. • This connective tissue supports each individual acinus and divides the gland into a series of lobes o lobules, finally encapsulating it.

Diagrammatic illustration of the ductal system of a salivary gland. • The main excretory duct, which empties into the oral cavity, divides into progressively smaller interlobar and interlobular excretory ducts that enter the lobes and lobules of the gland. • The predominant intralobularductalcomponent is the striated duct, which plays a major role in modification of the primary saliva produced by the secretoryend pieces. • Connecting the striated ducts to the secretory end pieces are intercalated ducts, which branch once or twice before joining individual end pieces. • The lumen of the end piece is continuous with that of the intercalated duct. Main excretory duct Excretory duct Striated duct Intercalated duct Canaliculus between cells Tubular secretory end piece Spherical secretory and piece

Intercellular canaliculi • In some glands, small extensions of the lumen, intercellular canaliculi, are found between adjacent secretory cells; • These intercellular canaliculi may extend almost to the base of the secretory cells and serve to increase the size of the secretory (luminal) surface of the cells. Lu N

Acini - the basic histologic structure of the major salivary glands is similar • Acini in the parotid glands are almost exclusively of the serous type, while those in the sublingual glands are predominantly mucous cells. • In the submaxillary glands, it is common to observe acini composed of both serous and mucous epithelial cells.

SECRETORY CELLS • Serous and mucous cells differ in structure and in the types of macromolecular components that they produce and secrete.

Serous cells • In general, serous cells produce proteins and glycoproteins (proteins modified by the addition of sugar residues [glycosylation]), many of which have well-defined enzymatic, antimicrobial, calciumbinding, or other activities. • Typically, serous glycoproteins have N-linked (bound to the β-amide of asparagine) oligosaccharide side chains.

Serous cell • Secretory end pieces that are composed of serous cells are typically spherical and consist of 8 to 12 cells surrounding a central lumen; • The cells are pyramidal, with a broad base adjacent to the connective tissue stroma and a narrow apex forming part of the lumen of the end piece. • The lumen usually has fingerlike extensions located between adjacent cells called intercellular canaliculi that increase the size of the luminal surface of the cells. • The spherical nuclei are located basally, and occasionally, binucleated cells are seen. • Numerous secretory granules, in which the macromolecular components of saliva are stored, are present in the apical cytoplasm Intercellular canaliculi are seen in longitudinal (right) and cross section (left).

Mucous cells • The main products of mucous cells are mucins, which have a protein core (apomucin) that is organized into specific domains and is highly substituted with sugar residues. • Mucinsare therefore also glycoproteins, but they differ from most serous cell glycoproteins in the structure of the protein core. • Mucinsfunction mainly to lubricate and form a barrier on surfaces and to bind and aggregate microorganisms. • Mucous cells secrete few, if any, other macromolecular components.

Mucous Cells • Secretory end pieces that are composed of mucous cells typically have a tubular configuration; • When cut in cross section, these tubules appear as round profiles with mucous cells surrounding a central lumen. Lu The nuclei (arrowheads) are flattened and compressed against the basal surfaces of the cells The lumina(Lu) are large compared with those of serous acini.

Mucous cell. • The most prominent feature of mucous cells is the accumulation in the apical cytoplasm of large amounts of secretory product (mucus), which compresses the nucleus an endoplasmic reticulum against the basal cell membrane.

Distinction between serous cells and mucous cells • In recent years the distinction between serous cells and mucous cells has become somewhat blurred. • Serous cells of some salivary glands are known to produce certain type of mucins, and some mucous cells are thought to produce certain nonglycosylated proteins.

Submandibular gland • Contain mucous cells coated with serous cells called – crescent; • Serous fluid passes down through the channels between terminal mucinous cells to the lumen of the alveoli.

Ducts • Within the ducts, the composition of the secretion is altered. • Much of the sodium is actively reabsorbed, potassium is secreted, and large quantities of bicarbonate ions are secreted.

Striatedduct

DUCTS • The ductal system of salivary glands is a varied network of tubules that progressively increase in diameter, beginning at the secretory end pieces and extending to the oral cavity; • The three classes of ducts are: • intercalated, • striated, • and excretory, each with differing structure and function. • The ductal system is more than just a simple conduit for the passage of saliva; • It actively participates in the production and modification of saliva. Excretory duct striated, • intercalated

Intercalated Ducts • The primary saliva produced by the secretory end pieces passes first through the intercalated ducts; • The first cells of the intercalated duct are directly adjacent to the secretory cells of the end piece, and the lumen of the end piece is continuous with the lumen of the intercalated duct. • The intercalated ducts are lined by a simple cuboidalepithelium,andmyoepithelial cell bodies and their processes. • The intercalated ducts contribute macromolecular components, which are stored in their secretorygranules, to the saliva. • These components include lysozyme and lactoferrin;

Intercalated duct cell • The intercalated duct cells have centrally placed nuclei and a small amount of cytoplasm containing some rough endoplasmic reticulum and a small Golgi complex • A few small secretory granules may be found in the apical cytoplasm, especially in cells located near the end pieces. • The apical cell surface has a few short microvilliprojecting into the lumen; • The lateral surfaces are joined by apical junctional complexes and scattered desmosomes and gap junctions and have folded processes that interdigitatewith similar processes of adjacent cells.

Striated Ducts • The striated ducts, which receive the primary saliva from the intercalated ducts, constitute the largest portion of the duct system. • These ducts are the main ductal component located within the lobules of the gland, that is, intralobular

striated ducts • The ducts have large lumina(Lu) and are lined by a pale-staining, simple columnar epithelial cells with centrally placed nuclei and faint basal striations. Lu SD

Striated duct cells • The basal striations of the duct cells are result from the presence of numerous elongated mitochondria, separated by highly infolded and interdigitatedbasolateral cell membranes; • The apical cytoplasm may contain small secretory granules and electron-lucent vesicles; • The granules contain kallikrein and perhaps other secretory proteins; • The presence of vesicles suggests that the cells may participate in endocytosis of substances from the lumen; • The duct cells contain numerous lysosomes and deposits of glycogen frequently are present in the perinuclear cytoplasm. • Adjacent cells are joined by tight junctions and junctional complexes but lack gap junctions.

Excretory Ducts • The excretory ducts are located in the connective tissue septa between the lobules of the gland, that is, in an extralobular or interlobular location. • These ducts are larger in diameter than striated ducts and typically have a pseudostratifiedepithelium with columnar cells extending from the basal lamina to the ductal lumen and small basal cells that sit on the basal lamina but do not reach the lumen.

Larger excretory duct • As the smaller ducts join to form larger excretory ducts, the number of basal cells increases, and scattered mucous cells may be present; • A large excretory duct is surrounded by dense connective tissue. • The pseudostratified epithelium contains several mucous goblet cells (arrowheads).

FORMATION AND SECRETION OF SALIVA • The formation of saliva occurs in two stages. • In the first stage, cells of the secretory end pieces and intercalated ducts produce primary saliva, which is an isotonic fluid containing most of the organic components and all of the water that is secreted by the salivary glands. • In the second stage, the primary saliva is modified as it passes through the striated and excretory ducts, mainly by reabsorption and secretion of electrolytes. • The final saliva that reaches the oral cavity is hypotonic.

DUCTAL MODIFICATION OF SALIVA • An important function of the striated and excretory ducts is the modification of the primary saliva produced by the end pieces and intercalated ducts occurring principally through reabsorption and secretion of electrolytes. • The luminal and basolateral membranes have abundant transporters that function to produce a net reabsorptionof Na+ and Cl– resulting in the formation of hypotonic final saliva. • The ducts also secrete K+ and HCO3but little if any secretion or reabsorption of water occurs in the striated and excretory ducts. Interstitum Na+ K+ Cl− H+ Na+ ATP K+ Striated duct cell Cl− Na+ Cl− Na+ TJ H+ H2O Lumen HCO3−

Salivary Glands and Saliva