The salivary glands are a collection of somewhat dissimilar structures in the mouth that produce a common j uice (the saliva), although the composition of the secretion from diﬀerent salivary glands varies. The largest of the salivary structures are the paired parotid glands, located near the angle of the j aw and the ear. They are serous glands, secrete a fairly watery j uice rich in amylase, and account for approximately 25% of the daily output of saliva. The smaller bilateral submandibular and sublingual glands are mixed glands, containing both serous and mucous cells. They elaborate a more viscid saliva and secrete most of the remaining 75% of the output. Other, still smaller glands are present in the mucosa covering the palate, buccal areas, lips, and tongue. Most of the salivary glands are ectodermal in origin. The combined secretion of the parotid and submandibular glands constitutes 90% of the volume of saliva, which in a normal adult can amount to a liter daily. The speciﬁc gravity of this mixed j uice ranges from 1.000 to 1.010. The microscopic structure of the salivary glands combines many features observed in another exocrine gland, the pancreas. A salivary gland consists of a blind-end system of microscopic ducts that branch out from grossly visible ducts. One main duct opens into the mouth from each gland. The functional unit of the salivary duct system, the salivon, is depicted in. At the blind end is the acinus, surrounded by polygonal acinar cells. These cells secrete the initial saliva, including water, electrolytes, and organic molecules such as amylase. The solutes and ﬂuid move out of the acinar cell into the duct lumen to form the initial saliva. The acinar cells are surrounded in turn by myoepithelial
cells. The myoepithelial cells rest on the basement membrane of acinar cells. They contain an actinomycin and have motile extensions. The next segment of the salivon is t h e intercalated duct, which may be associated with additional myoepithelial cells. Contraction of myoepithelial cells serves to expel formed saliva from the acinus, oppose retrograde movement of the j uice during active secretion of saliva, shorten and widen the internal diameter of
would permit back-diﬀusion of formed saliva through the stretched surface of the acinus). Thus the myoepithelial cells support the acinus against the increased intraluminal pressures that occur during secretion. Whenever there is an abrupt need for saliva in the mouth (e.g., immediately before vomiting), myoepithelial cell contraction propels the secretion into the main duct of the gland. Other exocrine glands, such as the mammary glands and the pancreas, also possess myoepithelial cells. The intercalated duct soon widens to become the striated duct, lined by columnar epithelial cells that resemble the epithelial components of the renal tubule in both shape and function. The saliva in the intercalated duct is similar in ionic composition to plasma. Changes from that composition occur because of ion exchanges in the striated duct. As saliva traverses the striated duct, sodium (Na+) is actively reabsorbed from the juice, and potassium (K+) is transported into it. Ca2+ also enters secreting duct cells during salivation. Similarly, anionic exchange occurs; chloride (Cl−) is reabsorbed from the saliva, and bicarbonate is added to it. The striated duct epithelium is considered to be a fairly “tight” sheet membrane. That is, its surface is fairly impermeable to the back-diﬀusion of water from the saliva into the tissue, and osmotic gradients develop between the saliva and interstitial ﬂuid through the reabsorption of Na+ and Cl−. The result is hypotonic saliva.
The blood supplied to the salivary glands is distributed by branches of the external carotid artery. The direction of arterial ﬂow within each salivary gland is opposite the direction of ﬂowing saliva within the ducts of each salivon. The arterioles break up into capillaries around the acini, as well as in nonacinar areas. Blood from nonacinar areas passes through portal venules back to the acinar capillaries, from which a second set of venules then drains all the blood to the systemic venous circulation. The rate of blood ﬂow through resting salivary tissue is approximately 20 times that through muscle. This blood ﬂow in part accounts for the prodigious amounts of saliva produced relative to the weight of the glands. Both components of the ANS reach the salivary glands. The parasympathetic preganglionic ﬁbers are delivered by the facial and glossopharyngeal nerves to autonomic ganglia, from which the postganglionic ﬁbers pass to individual glands. The sympathetic preganglionic nerves originate at the cervical ganglion, whose postganglionic ﬁbers extend to the glands in the periarterial spaces. Parasympathetic and sympathetic mediators regulate all known salivary gland functions to an extraordinary degree. Their inﬂuence includes maj or eﬀects, not only on secretion but also on blood ﬂow, ductular smooth muscle activity, growth, and metabolism of the salivary glands.