Salivary Glands

The salivary glands are exocrine glands positioned in and around the oral cavity. These glands are responsible for secreting saliva into the mouth, which aids in digestion. Saliva helps keep the oral mucosa lubricated, and it provides antimicrobial protection. There are 3 major paired salivary glands: the sublingual, submandibular, and parotid glands. There are also hundreds of minor salivary glands found in patches around the oral cavity.

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Introduction

The 3 pairs of major salivary glands are the parotid, submandibular, and sublingual glands.

Embryonic development

  • The parotid gland arises in weeks 5–6 from the ectoderm. 
  • The submandibular gland arises in weeks 6–7 from the endoderm. 
  • The sublingual gland arises in weeks 7–8 from the endoderm.
  • Intraoral minor salivary glands develop during the 3rd month from the endoderm.

Function

Production of saliva, which is important in:

  • Facilitating initiation of digestion
  • Lubricating and protecting the oral cavity
  • Exerting antimicrobial action
  • Enhancing taste

Morphology

  • The salivary glands are exocrine glands that secrete saliva into the oral cavity through ducts.
  • The glands, based on their size (the 1st is the largest) are:
    • Parotid 
    • Submandibular
    • Sublingual
    • Minor salivary glands
Overview of the salivary glands

Overview of the salivary glands and surrounding anatomic landmarks

Image by Lecturio.

Histology

  • The salivary glands are divided by connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue septa into lobes, then lobules, and finally acini (round secretory units) with ducts.
  • Secretion of saliva occurs in the acinus and the initial part of the duct system.
  • Acini:
    • Consist of:
      • Luminal cells: line the lumen and produce saliva
      • Myoepithelial cells: surround the luminal cells and contract on stimulation to constrict the acini
    • Secretions can be serous, mucinous, or mixed.
      • Serous (watery) secretions are produced mainly from the parotid gland.
      • Mucous secretions are produced from the minor glands. 
      • Mixed secretions are released from the sublingual and submandibular glands.
  • The salivary ducts are lined by 3 types of cells:
    • Intercalated cells:
      • Connect acini to the rest of the gland
      • Not involved in the modification of electrolytes Electrolytes Electrolytes are mineral salts that dissolve in water and dissociate into charged particles called ions, which can be either be positively (cations) or negatively (anions) charged. Electrolytes are distributed in the extracellular and intracellular compartments in different concentrations. Electrolytes are essential for various basic life-sustaining functions. Electrolytes
    • Striated cells:
      • Regulate electrolyte transport
      • Sodium resorption
    • Excretory duct cells:
      • Continuing sodium resorption
      • Secrete potassium
Table: Distinguishing features between the major salivary glands
Parotid Submandibular Sublingual
Location Near the ear Below the mandible Below the tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue
Development Ectodermal Endodermal Endodermal
Size Largest (15–30 g) Smaller (10–20 g) Smallest (3–4 g)
Shape Pyramidal J-shaped Almond-shaped
Duct and opening site Stensen duct:
opens opposite
2nd maxillary
molar
Wharton duct:
opens at the
sublingual papilla
(lateral to each side of
the frenulum linguae)
Series of ducts:
floor of the
oral cavity on
sublingual fold
Nerve supply Lesser petrosal nerve Chorda tympani Chorda tympani
Type of secretion Serous Serous and mucus Mucus (mainly)
and serous

Composition and Function of Saliva

Description and production

  • Saliva is a clear, slightly acidic mucoserous exocrine secretion that is critical to the preservation and maintenance of oral health.
  • Production: 0.5 to 1.5 L/day
  • Contributions of the different salivary glands during unstimulated flow are as follows:
    • 20% from parotid 
    • 65% from submandibular 
    • 7%–8% from sublingual 
    • < 10% from the minor glands
  • The parotid contributes > 50% of stimulated saliva flow.

Composition

  • Water: 99.5%
  • Electrolytes, including Na+, K+, Ca, Mg, bicarbonate (HCO3), and phosphates
  • Immunoglobulins Immunoglobulins Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins, proteins, enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes, mucins, urea, and ammonia
  • Normal pH: 6–7, slightly alkaline

Flow

  • Normal flow for unstimulated saliva: ≥ 0.1 mL/min.
  • On stimulation, ↑ volume to 0.2 mL/min
  • Saliva:
    • Flows from the acini into the ducts and eventually into the openings (in the mouth)
    • Composition is modified as saliva flows, adding K+ and HCO3 and extracting Na+ and chloride (Cl).
    • Extraction of NaCl makes saliva generally hypotonic. 
  • Controlled by a salivary center composed of nuclei in the medulla
  • The various stimuli for saliva production include:
    • Mechanical factors such as mastication (chewing)
    • Gustatory (e.g., acidic foods)
    • Olfactory
    • Psychological factors (e.g., pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain
    • Medications 
    • Localized or systemic diseases
Ion movement during production of saliva

Ion movement during production of saliva and its transports through the salivary duct

Image by Lecturio.

Function

  • Lubricates food, aiding swallowing
  • Starts digestion of carbohydrates Digestion of carbohydrates Carbohydrates store energy and are used as a source of nutrition. To be used as energy by humans, most carbohydrates must be metabolized. Carbohydrate metabolism involves transforming complex starches into glucose, a monosaccharide that can subsequently be absorbed by the body. Digestion and Absorption of Carbohydrates and lipids
  • Antibacterial activity through:
    • Secretory IgA: the largest immunologic component of saliva
    • Antibacterial enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes (e.g., lactoferrin)
    • Other antibacterial proteins that cause bacterial clumping
  • Protection of the oral mucosa and teeth Teeth Normally, an adult has 32 teeth: 16 maxillary and 16 mandibular. These teeth are divided into 4 quadrants with 8 teeth each. Each quadrant consists of 2 incisors (dentes incisivi), 1 canine (dens caninus), 2 premolars (dentes premolares), and 3 molars (dentes molares). Teeth are composed of enamel, dentin, and dental cement. Teeth
  • Aids in the sense of taste

Sublingual Gland

  • The smallest major salivary gland
  • Location: under the tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue, lateral to the genioglossus muscle, above the submandibular glands
  • Drainage: 
    • Has no single dominant duct, but is drained by 10 small ducts, called the ducts of Rivinus
    • Occasionally, some anterior ducts may join to form a large common duct, called the Bartholin duct.
    • The Bartholin duct typically joins the Wharton (submandibular) duct, which drains into the sublingual caruncle.
    • Other sublingual ducts empty directly into the bottom of the oral cavity.
  • Structure: 
    • Unencapsulated
    • Consists mainly of mucus acini, with just a few serous acini
  • Neurovasculature:
    • Blood supply: sublingual and submental arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
    • Venous return: sublingual vein
    • Lymphatic drainage: submandibular lymph nodes
    • Nerve supply (similar to submandibular gland):
      •  Chorda tympani nerve, a branch of the facial nerve 
      • Synapses at the submandibular ganglion
The sublingual and submandibular glands

The sublingual and submandibular glands, with their surrounding structures

Image by Lecturio.

Submandibular Gland

Location

  • Occupies most of the submandibular triangle
  • Boundaries of the triangle are:
    • Superior: inferior border of mandible
    • Lateral: posterior belly of digastric muscle
    • Medial: anterior belly of digastric muscle
    • Floor: mylohyoid and hyoglossus 
    • Roof: skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin, fascia, platysma

Structure

  • Divided into superficial and deep lobes, which are separated by the mylohyoid muscle
  • J-shaped, with a larger and a smaller part of the gland:
    • Superficial lobe:
      • Larger
      • Within the anterior part of triangle
      • Outside the oral cavity
    • Deep lobe:
      • Smaller: curves posteriorly, connecting to superficial portion of gland (forming a “J”)
      • Located on the lateral side of the hyoglossus muscle
      • Inside the oral cavity
  • The submandibular, or Wharton, duct empties on both sides of the frenulum at the sublingual caruncle.
    • Prior to reaching the opening, the duct crosses paths with the lingual nerve.
    • Cranial nerve (CN) XII (hypoglossal nerve) runs parallel and inferior to the Wharton duct.
  • Consists mainly of serous acini, with few mucinous acini

Neurovasculature

  • Blood supply: sublingual and submental arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
  • Venous return: common facial vein and sublingual vein
  • Lymphatic drainage: submandibular nodes → upper deep cervical nodes
  • Nerve supply: 
    • Parasympathetic (secretomotor):
      • Chorda tympani, a branch of the facial nerve
      • Parasympathetic cell bodies in the submandibular ganglion
    • Sympathetic (vasoconstriction): from the superior cervical ganglion
Submandibular gland and spatial relations

Submandibular gland and associated neurovascular structures

Image by Lecturio.

Parotid Gland

Location

Located in the retromandibular fossa, the parotid gland:

  • Wraps around the mandibular ramus
  • Covers the posterior portion of the masseter muscle
  • Sits inferior to the zygomatic arch

Structure

  • Has 2 lobes, a broad superficial lobe and a smaller deeper lobe, with the facial nerve running between them.
    • The superficial lobe is lateral to the facial nerve.
    • The deeper lobe is medial to the facial nerve.
  • The main duct, called the Stensen duct, is located opposite to the 2nd maxillary molar and empties into the oral cavity.
  • Consists mostly of serous acini
Histology of the parotid gland

Histology of the parotid gland (serous acini)

Image: “Histology of the parotid gland” by Wbensmith. License: CC BY 3.0

Spatial relations

  • Posterior: 
    • Sternocleidomastoid muscle
    • Mastoid process
  • Anterior:
    • Angle of mandible
    • Masseter
    • Medial pterygoid plate
  • Superior: zygomatic arch
  • Within the gland:
    • Facial nerve
    • Retromandibular vein
    • External carotid artery
    • Superficial temporal artery
    • Fibers of auriculotemporal nerve
The structures within the parotid gland

The facial nerve (top image) and its branches (lower image) pierce the parotid gland

Image by Lecturio.

Neurovasculature

  • Blood supply:
    • External carotid artery and its terminal branches
    • Venous return: retromandibular vein
    • The vascular bundle ( arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries and veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins) lies medial to the facial nerve
  • Lymphatic drainage: 2 nodal layers (extraglandular and intraglandular nodes) draining into the deep and superficial cervical lymph systems
  • Innervation: 
    • Parasympathetic innervation via the lesser petrosal nerve (a component of the glossopharyngeal nerve (CN IX))
    • Parasympathetic cell bodies are in the otic ganglion, with postganglionic fibers joining the auriculotemporal nerve.
Innervation, blood and venous supply of parotid gland

Facial nerve and branches with the arterial and venous structures of the parotid gland

Image by Lecturio.

Minor Salivary Glands

  • Hundreds of minor glands are present.
  • Highest concentration is noted in the buccal mucosa, labial mucosa, lingual mucosa, soft/hard palate Palate The palate is the structure that forms the roof of the mouth and floor of the nasal cavity. This structure is divided into soft and hard palates. Oral Cavity: Palate, and floor of mouth. 
  • Produce < 10% of saliva, with mainly mucous secretions

Clinical Relevance

  • Diseases of the salivary glands Diseases of the salivary glands Diseases of the salivary glands include sialadenosis, sialadenitis, sialolithiasis, and neoplasms. Sialadenosis is a chronic, bilateral, noninflammatory hypertrophy of the salivary glands. Sialadenitis results from inflammation or infection of the glands, and sialolithiasis is due to stone formation in the glands or ducts. Diseases of the Salivary Glands: Some conditions manifest as salivary gland enlargement with various degrees of pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain or discomfort. Specific clinical factors help differentiate between these conditions, such as asymmetry, the presence of pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain, associated conditions and risk factors, and growth or stability. The diagnosis is often clinical, though imaging and biopsy may be needed. Management varies depending on the disease.
    • Sialadenosis is chronic, bilateral, noninflammatory hypertrophy of the salivary glands. 
    • Sialadenitis results from inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation or infection of the glands.
    • Sialolithiasis is due to stone formation in the glands or ducts. 
    • Neoplasms of the salivary gland may be benign or malignant.
  • Mumps Mumps Mumps is caused by a single-stranded, linear, negative-sense RNA virus of the family Paramyxoviridae. Mumps is typically a disease of childhood, which manifests initially with fever, muscle pain, headache, poor appetite, and a general feeling of malaise, and is classically followed by parotitis. Mumps Virus/Mumps: typically a disease of childhood. Mumps Mumps Mumps is caused by a single-stranded, linear, negative-sense RNA virus of the family Paramyxoviridae. Mumps is typically a disease of childhood, which manifests initially with fever, muscle pain, headache, poor appetite, and a general feeling of malaise, and is classically followed by parotitis. Mumps Virus/Mumps is caused by an RNA RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology: Overview of the family Paramyxoviridae. Transmission is via respiratory droplets or direct contact. The condition manifests initially with fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, muscle pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain, headache, poor appetite, and general malaise, classically followed by parotitis. Complications include meningitis Meningitis Meningitis is inflammation of the meninges, the protective membranes of the brain, and spinal cord. The causes of meningitis are varied, with the most common being bacterial or viral infection. The classic presentation of meningitis is a triad of fever, altered mental status, and nuchal rigidity. Meningitis, pancreatitis, permanent deafness, and testicular inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation, which can result in infertility Infertility Infertility is the inability to conceive in the context of regular intercourse. The most common causes of infertility in women are related to ovulatory dysfunction or tubal obstruction, whereas, in men, abnormal sperm is a common cause. Infertility. Management is with supportive care. Mumps Mumps Mumps is caused by a single-stranded, linear, negative-sense RNA virus of the family Paramyxoviridae. Mumps is typically a disease of childhood, which manifests initially with fever, muscle pain, headache, poor appetite, and a general feeling of malaise, and is classically followed by parotitis. Mumps Virus/Mumps is prevented by vaccination Vaccination Vaccination is the administration of a substance to induce the immune system to develop protection against a disease. Unlike passive immunization, which involves the administration of pre-performed antibodies, active immunization constitutes the administration of a vaccine to stimulate the body to produce its own antibodies. Vaccination.
  • Ranula: mucous-retention cyst of the sublingual gland or small salivary glands present in the floor of the mouth. A ranula is a benign swelling that is soft and bluish and transilluminates.
  • Sjögren syndrome: autoimmune, inflammatory condition in which glandular tissues such as the salivary and lacrimal glands are infiltrated by lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes, resulting in decreased tear and saliva production. Symptoms include dry eyes and mouth, accompanied by extraglandular manifestations. Diagnosis is via history and clinical examination, serologic studies, or salivary gland biopsy.  A multidisciplinary approach is needed to treat patients.

References

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  2. Chason, H.M., Downs, B.W. (2021). Anatomy, head and neck, parotid gland. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK534225/
  3. Del Vigna de Almeida, P., et al. (2008). Saliva composition and functions: a comprehensive review. J Contemp Dent Pract 9(3):72–80.
  4. Grewal, J.S., Jamal, Z., Ryan, J. (2020) Anatomy, Head and Neck, Submandibular Gland. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK542272/
  5. Grewal, J.S., et al. (2021) Anatomy, head and neck, sublingual gland. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK535426/
  6. Gupta, S., Ahuja, N.  (2019). Salivary glands. IntechOpen. https://www.intechopen.com/chapters/63843
  7. ​Humphrey, S. P., Williamson, R. T. (2001). A review of saliva: normal composition, flow, and function. Journal of Prosthetic Dentistry 85(2):162–169.
  8. Iorgulescu, G. (2009) Saliva between normal and pathological: important factors in determining systemic and oral health. Journal of Medicine and Life 2:303–307.
  9. Kessler, A. T., Bhatt, A. A. (2018). Review of the major and minor salivary glands, part 1: anatomy, infectious, and inflammatory processes. Journal of Clinical Imaging Science 8:47. https://doi.org/10.4103/jcis.JCIS_45_18
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