Salivary Glands Flashcards
INTRODUCTION
- WHAT IS SALIVA
The oral cavity is kept moist by a film of fluid called saliva that coats the
teeth and the mucosa.
Saliva is a complex fluid, produced by the salivary
glands.
2.TYPES
In human beings, three pairs of major salivary glands
—the parotid,
submandibular, and sublingual—are located outside the oral cavity, with
extended duct systems through which the gland secretions reach the
mouth.
Numerous smaller minor salivary glands are located in various
parts of the oral cavity—the labial, lingual, palatal, buccal,
glossopalatine, and retromolar glands—typically located in the
submucosal layer
with short ducts opening directly onto
the mucosal surface.
The parotid glands
secrete a watery saliva rich in enzymes
such as amylase,
proteins such as the proline-rich proteins, and
glycoproteins.
Submandibular saliva, in addition to the components contains highly glycosylated substances called mucins.
The
sublingual gland produces viscous saliva also rich in mucins.
Oral fluid,
which is referred to as mixed, or whole, saliva, includes the secretions of
- the major glands,
- the minor glands,
- desquamated oral epithelial cells,
- microorganisms and their products,
- food debris,
- serum components
- inflammatory cells that gain access through the gingival crevice.
Moreover, whole saliva is not the simple sum of all of these components
because many of the proteins are removed as they adhere to the surfaces
COMPOSITION OF SALIVA
Parameter &
Characteristics
1. Volume
- 600–1000 mL/day
2. Electrolytes
Na+, K+, Cl−, Ca2+,Mg2+,
,SCN−, and F−
- Secretory
proteins/peptides
- Amylase,
- proline-rich proteins
- mucins,
- histatin,
- cystatin,
- peroxidase,
- lysozyme,
- lactoferrin,
- defensins, and
- cathelicidin-LL37
-
Immunoglobulins
- Secretory immunoglobulin A;
- immunoglobulins G and M -
Small organic
- Glucose,
- amino acids,
- urea,
- uric acid,
- lipid molecules - Other
components
- Epidermal growth factor,
- insulin,
- cyclic adenosine
- monophosphate–binding
proteins, - serum albumin
FUNCTIONS OF SALIVA
-
Protection
- Saliva protects the oral cavity in many ways.
- The fluid nature of saliva
provides a washing action that flushes away non adherent bacteria and
other debris.
- In particular, the clearance of sugars from the mouth limits
their availability to acidogenic plaque microorganisms.
- The mucins and
other glycoproteins provide lubrication, preventing the oral tissues from
adhering to one another and allowing them to slide easily over one
another. - The mucins also form a barrier against noxious stimuli,
microbial toxins, and minor trauma.
-
Buffering
- The bicarbonate and, to some extent, phosphate, ions in saliva provide a
buffering action that helps protect the teeth from demineralization
caused by bacterial acids produced during sugar metabolism.
- Some basic
salivary proteins also may contribute to the buffering action of saliva.
Additionally, the metabolism of salivary proteins and peptides by
bacteria produces urea and ammonia, which help to increase the pH.
- Pellicle Formation
- Many of the salivary proteins bind to the surfaces of the teeth and oral
mucosa, forming a thin film, the salivary pellicle. - Several proteins bind
calcium and help to protect the tooth surface. - Others have binding sites
for oral bacteria, providing the initial attachment for organisms that form
plaque.
- Maintenance of Tooth Integrity
- Saliva is supersaturated with calcium and phosphate ions.
The solubility
of these ions is maintained by several calcium-binding proteins,
especially the acidic proline-rich proteins and statherin. - At the tooth
surface the high concentration of calcium and phosphate results in a
posteruptive maturation of the enamel, increasing surface hardness and
resistance to demineralization. -
Remineralization of initial caries lesions
also can occur; this is enhanced by the presence of fluoride ions in saliva.
- Antimicrobial Action
- Saliva has a major ecologic influence on the microorganisms that colonize
oral tissues. - In addition to the barrier effect provided by mucins, saliva
contains a spectrum of proteins with antimicrobial activity such as the
lysozyme, lactoferrin, peroxidase, and secretory leukocyte protease
inhibitor. - A number of small peptides that function by inserting into
membranes and disrupting cellular or mitochondrial functions are
present in saliva.
These include α-defensins and β-defensins,
cathelicidin-LL37, and the histatins. - In addition to antibacterial and
antifungal activities, several of these proteins and peptides also exhibit
antiviral activity.
The major salivary immunoglobulin, secretory
(IgA),
causes
agglutination
of
specific
microorganisms, preventing their adherence to oral tissues and forming
clumps that are swallowed.
- Mucins, as well as specific agglutinins, also
aggregate microorganisms.
- Tissue Repair
- A variety of growth factors and other biologically active peptides and
proteins are present in small quantities in saliva. - Under experimental
conditions, many of these substances promote tissue growth and
differentiation, wound healing, and other beneficial effects. However, the
role of most of these substances in protection of the oral cavity is
presently unknown.
-
Digestion
- Saliva also contributes to the digestion of food.
- The solubilization of food
substances and the actions of enzymes such as amylase and lipase begin
the digestive process.
- The moistening and lubricative properties of saliva
also allow the formation and swallowing of a food bolus.
-
Taste
- Saliva functions in taste by solubilizing food substances so that they can
be sensed by taste receptors located in taste buds.
- Saliva produced by
minor glands in the vicinity of the circumvallate papillae contains
proteins that are believed to bind taste substances and present them to
the taste receptors. - Additionally, saliva contains proteins that have a
trophic effect on taste receptors.
ANATOMY OF SALIVARY GLANDS
Anatomy
- 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 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. - A small amount of parotid
tissue occasionally forms an accessory gland associated with stensons duct, just anterior to the superficial portion. - The parotid gland receives
its blood supply from branches of the external carotid artery as they pass
through the gland. -
The parasympathetic nerve supply to the parotid
gland is mainly from the glossopharyngeal nerve (cranial nerve IX). - The
preganglionic fibers synapse in the otic ganglion; the postganglionic
fibers reach the gland through the auriculotemporal nerve. - The
sympathetic innervation of all of the salivary glands is provided by
postganglionic fibers from the superior cervical ganglion, traveling with
the blood supply. - 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 mylohyoid muscle
- The excretory duct (Wharton’s duct) of the
submandibular gland runs forward above the mylohyoid muscle 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 (cranial nerve VII),
reaching the gland through the lingual nerve and submandibular
ganglion. - 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 (cranial nerve VII) provides the parasympathetic innervation
of the sublingual gland, also via the lingual nerve and submandibular
ganglion. - The minor salivary glands,
- 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 gingiva and 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.
STRUCTURE OF SALIVARY GLANDS
STRUCTURE OF TERMINAL SECRETORY UNITS (ACINI)
The basic functional unit of a salivary gland is the termi- nal secretory unit called acini.
The terminal secretory unit, irrespective of size and location, is made up of epithelial secretory cells, namely serous and mucous cells.
The serous, mucous along with myoepithelial (ME) cells are arranged in an acinus or acini (multiple) with a roughly spherical or tubular shape and a central lumen
The cells in the acini rest on a basement membrane.
They are arranged in a single layer.
The intercellular spaces of the apical ends of the cells are separated from the lumen by junctional complexes that are tight (zonula occludens), intermediate junction (zonula adherens). and one or more desmosomes (maculae adherens).
The junctional complexes
- hold the cells together in an acinus
- regulate the permeability.
Tight junctions
- seal the adjacent secretory cells, - controlling paracellular ion influx. - This helps in maintaining cell polarity and tissue homeostasis.
The main tight junctional proteins are
- claudin,
- occludin,
- and junctional adhesion molecules.
The ME cells are located on the surface of the acini.
The central lumen of each acinus may have a star shaped morphology because of extension of lumen in between the cells called intercellular canaliculi.
The central lumen of the acini continue via a fine series of ducts which constantly merge with each other and grow, larger eventually to merge into the main excretory duct.
These ducts comprise the ductal system.
The mucous acini have a larger lumen than serous acini (end piece).
The secretory terminal unit in serous cells is generally made of 8-12 serous acini surrounding a central lumen
Secretory end piece of mucous acini have a tubular configuration. The mucous cells are joined to each other by a variety of intracellular junctions, but unlike the serous acini, they lack the presence of intercellular cana- liculi.
The intercellular canaliculi are said to be present only in acini with demilunes
Sometimes mucous acini have bonnet or crescent-shaped covering, which is made of serous cells. These are called demilunes
The presence of demilunes is questioned. It has been shown that demilunes are as a result of artifact during tissue preparation.
Recent methods, like rapid freezing, freeze substitution and three dimensional reconstruction, have shown that serous cells align with mucous cells to surround a common lumen
Development
-
Origins and Growth
- Salivary glands develop from oral epithelial cells.
- Epithelial cell proliferation forms a thickening that grows into ectomesenchyme.
- This growth results in a bud connected to the surface by trailing cord of epithelial cells, with mesenchymal cells condensing around the bud.
- Branching morphogenesis occurs where clefts develop in the bud, forming new buds.
-
Interactions and Factors
- Branching morphogenesis requires interactions between epithelium and mesenchyme.
- Signaling molecules like fibroblast growth factors, sonic hedgehog, and transforming growth factor β, along with their receptors, control branch points and gland structure.
- Differential contraction of actin filaments aids cleft formation, while extracellular matrix stabilizes clefts.
- Specific mesenchyme supports gland development.
-
Lumen Formation
- Lumen development occurs in a specific order within the branched epithelium.
- Lumina form first in distal ends and branch cords, then in proximal ends, and finally in the central portion.
- Lumen formation may involve apoptosis in cell cords.
-
Epithelial Differentiation
- After lumen formation, the epithelium has two layers.
- Inner layer cells differentiate into secretory cells (mucous or serous).
- Outer layer cells form myoepithelial cells around secretory end pieces and ducts.
SEROUS cell
Types of Secretory Cells:
- Serous cells and mucous cells are the two main types of secretory cells in salivary glands.
- They differ in structure and the macromolecular components they produce and secrete.
Serous Cells:
- Compose secretory end pieces, typically spherical with 8 to 12 cells surrounding a central lumen.
- Cells are pyramidal with a broad base adjacent to connective tissue and a narrow apex forming part of the end piece’s lumen.
- Finger-like extensions called intercellular canaliculi increase luminal surface area.
- Spherical nuclei are basally located; sometimes binucleated cells are seen.
- Apical cytoplasm contains numerous secretory granules storing macromolecular components of saliva.
- Granules vary in appearance from electron-dense to a combination of dense and lucent regions.
- Basal cytoplasm contains rough endoplasmic reticulum cisternae converging on a large Golgi complex.
- Specializations of plasma membranes include microvilli on luminal surface, folds on lateral surfaces, and basal folds increasing surface area.
- Intercellular junctions like tight junctions, adhering junctions, and desmosomes form a junctional complex regulating material passage and cell adhesion.
- Hemidesmosomes attach cells to basal lamina and connective tissue.
- Gap junctions along lateral surfaces allow passage of small molecules between cells and coordinate cell activity within an end piece.
.
Mucous cell
Mucous Cells:
- Compose secretory end pieces with a tubular configuration; in cross-section, tubules appear as round profiles with mucous cells surrounding a central, larger lumen compared to serous end pieces.
- In major and some minor salivary glands, mucous end pieces may have associated serous demilune or crescent cells covering them.
- The secretory product of mucous cells is mucus, which accumulates in large amounts in the apical cytoplasm, compressing the nucleus and endoplasmic reticulum against the basal cell membrane.
- In routine histologic preparations, the secretory material appears unstained, giving an empty appearance to the cytoplasm above the nucleus, but special stains reveal sugar residues or acidic groups.
- Mucous secretory granules appear swollen and disrupted in electron microscopy, often fused with one another, and may contain filamentous or flocculent material.
- Rapid freezing and subsequent electron microscopy show smaller, dense mucous secretory granules with intact membranes.
- Mucous cells have a large Golgi complex mainly basal to secretory granules, with granules forming at the trans face and joining the rest in the apical cytoplasm.
- Organelles like the endoplasmic reticulum are mainly limited to the basal cytoplasm.
- Mucous cells lack intercellular canaliculi except for those covered by demilune cells, unlike serous cells.
