Salivary gland

The salivary glands in mammals are exocrine glands that produce saliva. In other taxa such as insects, salivary glands are often used to produce biologically important proteins such as silk or glues. Salivary glands have proven to be very useful to students of genetics due to the occurrence of polytene chromosomes that are common in the salivary gland cells of many Diptera.

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Summary

A summary is provided in the following table.

Histology

The glands are enclosed in a capsule of connective tissue and internally divided into lobules. Blood vessels and nerves enter the glands at the hilum and gradually branch out into the lobules.

Cells

There are 3 main types of cells that are found in the major salivary glands:

1. Serous cells, which are pyramidal in shape and are joined to usually form a spherical mass of cells called acinus, with a small lumen in the centre. Serous demilunes are found in the submandibular gland.
2. Mucous cells are usually cuboid in shape and organised as tubules, consisting of cylindrical arrays of secretory cells surrounding a lumen. These cells produce glycoproteins that are used for the moistening and lubricating functions of saliva.
3. Myoepithelial cells surround each secretory portion and are able to contract to accelerate secretion of the saliva.

Ducts

In the duct system, the lumens formed by the secretory cells empty into intercalated ducts, which in turn join to form striated ducts. These drain into ducts situated between the lobes of the gland (called interlobar ducts or excretory ducts).

The main duct of the salivary glands ultimately empties into the mouth. Salivary glands release saliva that dilutes the acid found in ones stomach.

Innervation

Salivary glands are innervated, either directly or indirectly, by the parasympathetic and sympathetic arms of the autonomic nervous system.

• Parasympathetic innervation to the salivary glands is carried via cranial nerves. The parotid gland receives its parasympathetic input from the glossopharyngeal nerve (CN IX) via the otic ganglion, while the submandibular and sublingual glands receive their parasympathetic input from the facial nerve (CN VII) via the submandibular ganglion.

• Direct sympathetic innervation of the salivary glands takes place via preganglionic nerves in the thoracic segments T1-T3 which synapse in the superior cervical ganglion with postganglionic neurons that release norepinephrine, which is then received by β-adrenergic receptors on the acinar and ductal cells of the salivary glands, leading to an increase in cyclic adenosine monophosphate (cAMP) levels and the corresponding increase of saliva secretion. Note that in this regard both parasympathetic and sympathetic stimuli result in an increase in salivary gland secretions. Costanzo, L. (2006). Physiology, 3rd ed.. Saunders Elsevier. ISBN 10:1-4160-2320-8.  The sympathetic nervous system also affects salivary gland secretions indirectly by innervating the blood vessels that supply the glands.it is used to

Role in disease

See mumps (parotiditis epidemica), Sjögren's syndrome and Mucocele.

Salivary duct calculus may cause blockage of the ducts, causing pain and swelling of the gland.

Tumors of the salivary glands may occur. These are usually benign, but may be malignant. The most common type of benign tumor is pleomorphic adenoma, followed by Warthin's tumor. The most common malignant tumor is mucoepidermoid carcinoma.

Diagnostic investigation

A sialogram is a radiocontrast study of a salivary duct.

Additional images

References