The Sublingual Gland

EMBRYOLOGY

The sublingual gland arises in 20 mm embryos as a number of small epithelial thickenings in the linguogingival groove and on the outer side of the groove. Each thickening forms its own canal and so many of the sublingual ducts open directly onto the summit of the sublingual fold. Those that arise within the linguogingival groove end up draining into the submandibular duct.

ANATOMY

The sublingual gland is the smallest of the major salivary glands. It is almond shaped and weighs approximately 4 g. It is predominantly a mucous gland. The gland lies on the mylohyoid and is covered by the mucosa of the floor of the mouth, which is raised as it overlies the gland to form the sublingual fold. Posteriorly, the sublingual gland is in contact with the deep lobe of the submandib-ular gland. The sublingual fossa of the mandible is located laterally and the genioglossus muscle is located medially. The lingual nerve and the sub-mandibular duct lie medial to the sublingual gland between it and the genioglossus.

Sublingual Ducts

The gland has a variable number of excretory ducts ranging from 8 to 20. The majority drain into the floor of the mouth at the crest of the sublingual fold. A few drain into the submandibu-lar duct. Sometimes, a collection of draining ducts coalesce anteriorly to form a major duct (Bartholin's duct), which opens with the orifice of the submandibular duct at the sublingual papilla.

Blood Supply, Innervation, and Lymphatic Drainage

The arterial supply is from the sublingual branch of the lingual artery and also the submental branch of the facial artery. Innervation is via the sublingual ganglion as described above. The lymphatics drain to the submental nodes.

Minor Salivary Glands

Minor salivary glands are distributed widely in the oral cavity and oropharynx. They are grouped as labial, buccal, palatoglossal, palatal, and lingual glands. The labial and buccal glands contain both mucous and serous acini, whereas the palatoglossal glands are mucous secreting. The palatal glands, which are also mucous secreting, occur in both the hard and soft palates. The anterior and posterior lingual glands are mainly mucous. The anterior glands are embedded within the muscle ventrally and they drain via four or five ducts near the lingual frenum. The posterior lingual glands are located at the root of the tongue. The deep posterior lingual glands are predominantly serous. Additional serous glands (of von Ebner) occur around the circumvallate papillae on the dorsum of the tongue. Their watery secretion is thought to be important in spreading taste stimuli over the taste buds.

Histology of the Salivary Glands

The salivary glands are composed of large numbers of secretory acini, which may be tubular or globular in shape. Each acinus drains into a duct. These microscopic ducts coalesce to form lobular ducts. Each lobule has its own duct and these then merge to form the main ducts. The individual lobes and lobules are separated by dense connective tissue, which is continuous with the gland capsule. The ducts, blood vessels, lymphatics, and nerves run through and are supported by this connective tissue.

The acini are the primary secretory organs but the saliva is modified as it passes through the intercalated, striated, and excretory ducts before being discharged into the mouth and oropharynx (Figure 1.14). The lobules also contain significant amounts of adipose tissue particularly in the parotid gland. The proportion of adipose tissue relative to excretory acinar cells increases with age.

In the human parotid, the excretory acini are almost entirely serous. In the submandibular gland, again, the secretory units are mostly serous but there are additional mucous tubules and acini. In some areas the mucinous acini have crescentic "caps" of serous cells called serous demilunes. In the sublingual gland the acini are almost entirely mucinous, although there are occasional serous acini or demilunes.

The serous cells contain numerous pro-teinaceous secretory (zymogen) granules. These granules contain high levels of amylase. In addition, the secretory cells produce kallikrein, lactoferrin, and lysozyme. In mucous cells, the cytoplasm is packed with large pale secretory droplets.

Initially the secretory acini drain into intercalated ducts. These function mainly to conduct the saliva but they may also modify the electrolyte content and secrete immunoglobulin A. The intercalated ducts drain into striated ducts, which coalesce into intralobular and extralobular collecting ducts. The intercalated duct cells are very active metabolically and they transport potassium and bicarbonate into saliva. They reabsorb sodium and chloride ions so that the resulting saliva is hypotonic. They also secrete immunoglobulin A, lysozyme, and kallikrein. The immunoglobulin is produced by plasma cells adjacent to the striated duct cells and it is then transported through the epithelial lining into the saliva. The main collecting ducts are simple conduits for saliva and do not modify the composition of the saliva.

Myoepithelial cells are contractile cells closely related to the secretory acini and also much of the duct system. The myoepithelial cells lie between the basal lamina and the epithelial cells. Numerous cytoplasmic processes arise from them and surround the serous acini as basket cells. Those associated with the duct cells are more fusiform and are aligned along the length of the ducts. The cytoplasm of the myoepithelial cells contains actin myofilaments, which contract as a result of both parasympathetic and sympathetic activity. Thus the myoepithelial cells "squeeze" the saliva out of the secretory acini and ducts and add to the salivary secretory pressure.

Serous cell

Intercalated duct cell

Tight junction

Homogeneous ■ V electron-translucent secretory vesicles

-iHS

Flattened basal nucleus

Serous cell

Intercalated duct cell

Tight junction

Flattened basal nucleus

Prominent apical web of microfilaments

Process of myoepithelial cell

Junctions between cholinergic axons and intercalated duct cells

Myoepithelial cell

Complex arborization of adrenergic and cholinergic axons around secretory unit or 'endpiece'

Mucous secretory u^^ endpiece

Adrenergic nerve terminals

Figure 1.14. Diagram showing the histology of the major components of the salivary glands. Published with permission, Elsevier Churchill Livingstone, Oxford, Standring S, Editor in Chief, Gray's Anatomy, 39th edition.

Prominent apical web of microfilaments

Process of myoepithelial cell

Junctions between cholinergic axons and intercalated duct cells

Myoepithelial cell

Striated (intralobular) duct accompanied by nonmyelinated postganglionic axon

Complex arborization of adrenergic and cholinergic axons around secretory unit or 'endpiece'

To interlobular [Immun°globulin] excretory ducts

Mucous secretory u^^ endpiece

Adrenergic nerve terminals

Arteriole accompanied by adrenergic axons

Figure 1.14. Diagram showing the histology of the major components of the salivary glands. Published with permission, Elsevier Churchill Livingstone, Oxford, Standring S, Editor in Chief, Gray's Anatomy, 39th edition.

Control of Salivation

There is a continuous low background saliva production that is stimulated by drying of the oral and pharyngeal mucosa. A rapid increase in the resting levels occurs as a reflex in response to masticatory stimuli including the mechanoreceptors and taste fibers. Other sensory modalities such as smell are also involved. The afferent input is via the saliva-tory centers, which are themselves influenced by the higher centers. The higher centers may be facilitory or inhibitory depending on the circumstances. The efferent secretory drive to the salivary glands passes via the parasympathetic and sympathetic pathways. There are no peripheral inhibitory mechanisms.

Cholinergic nerves (parasympathetic) often accompany ducts and branch freely around the secretory endpieces (acini). Adrenergic nerves (sympathetic) usually enter the glands along the arteries and arterioles and ramify with them. Within the glands, the nerve fibers intermingle such that cholinergic and adrenergic axons frequently lie in adjacent invaginations of a single Schwann cell. Secretion and vasoconstriction are mediated by separate sympathetic axons, whereas a single parasympathetic axon may, through serial terminals, result in vasodilatation, secretion, and constriction of myoepithelial cells.

Secretory endpieces are the most densely innervated structures in the salivary glands. Individual acinar cells may have both cholinergic and adrenergic nerve endings. The secretion of water and electrolytes, which accounts for the volume of saliva produced, results from a complex set of stimuli that are largely parasympathetic. The active secretion of proteins into the saliva depends upon the relative levels of both sympathetic and para-sympathetic stimulation.

Although the ducts are less densely innervated than secretory acini, they do influence the composition of the saliva. Adrenal aldosterone promotes resorption of sodium and secretion of potassium into the saliva by striated ductal cells. Myoepithelial cell contraction is stimulated predominantly by adrenergic fibers, although there may be an additional role for cholinergic axons.

Summary

• Although embryologically the parotid consists of a single lobe, anatomically the facial nerve lies in a distinct plane between the anatomical superficial and deep lobes.

• There are fixed anatomical landmarks indicating the origin of the extracranial facial nerve as it leaves the stylomastoid foramen.

• The lower pole of the parotid gland is separated from the posterior pole of the sub-mandibular gland by only thin fascia. This can lead to diagnostic confusion in determining the origin of a swelling in this area.

• The relationship of the submandibular salivary duct to the lingual nerve is critical to the safe removal of stones within the duct.

• Great care must be taken to identify the lingual nerve when excising the submandibular gland. The lingual nerve is attached to the gland by the parasympathetic fibers synapsing in the submandibular (sublingual) ganglion.

• The sublingual gland may drain into the sub-mandibular duct or it may drain directly into the floor of the mouth via multiple secretory ducts.

References

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