Composition of saliva
Although composed mainly of water, saliva is a complex secretion. As discussed earlier, so-called whole saliva consists primarily of the secretions from the major and minor salivary glands. Whole saliva also contains a number of constituents of nonsalivary origin: crevicular fluid, serum, and blood cells; bacteria and bacterial products; desquamated epithelial cells and cellular components; viruses and fungi; food debris; fluoride; and some bronchial secretions. It is estimated that 1 mL of whole saliva contains about 700 million viable bacteria, 0.5 million leukocytes (more
than 90% polymorphonuclear neutrophil leukocyte-cells), thousands of desquamated epithelial cells, 2 mg of proteins, 800 mg of lipids, 100 mg of immunoglobulins, and some inorganic electrolytes, such as calcium, phosphates, bicarbonate, sodium, chloride, and fluoride. Even pure secretions collected directly from the orifices of the main excretory ducts of the parotid, submandibular, or sublingual glands contain the saliva synthesized by the secretory cells, along with certain substances supplied by the circulation.
The composition of whole saliva is influenced by a number of physiologic factors.
Important among these are the source, the method of collection, and the degree of stimulation. As described earlier, the major salivary glands are composed of different acinar cells, programmed to synthesize quite different secretions. The parotid glands have serous acinar cells and produce a proteinaceous, watery secretion. The secretion from the sublingual glands is mucous and hence more viscous. The submandibular glands have both serous and mucous acinar cells and produce a saliva with lower protein content and higher viscosity than do the parotid glands.
The minor salivary glands are purely mucous glands and produce a saliva that is particularly viscous and rich in secretory immunoglobulin A (IgA). In response to stimulation, there may be a manifold increase in salivary output, with significant changes in consistency and in the concentration of many of its constituents. About 99% of the saliva is water. The remaining 1% consists mainly of large organic molecules, (eg, proteins, glycoproteins, and lipids); small organic molecules (eg, glucose and urea); and electrolytes (chiefly sodium, calcium, chloride, and phosphates). Most of the organic molecules are produced by the acinar cells; some are synthesized in the ducts, and some are transported into the saliva from the blood. A list of salivary constituents, subclassified as proteins, small organic molecules, and electrolytes, is presented in alphabetical order in Box 12.
The major proteins of the salivary glands are produced by the acinar cells and exist as families. Each family has a number of distinct but closely related members (genetic polymorphism). They include the proline-rich proteins (with at least 13 discrete members); the histatins (histidine-rich proteins with five related components); the cystatins (cystine-containing proteins); the tyrosine-rich proteins (statherin and others); mucins of high and low molecular weight; glycosylated and nonglycosylated amylases; and several salivary peroxidases.
Other salivary proteins exist in a single form, some produced by acinar and some by ductal cells. Among the acinar proteins are epidermal growth factor, secretory component, and lactoferrin. Lysozyme is known to be produced by duct cells, but, for many other constituents, the site of origin is unknown. Included among the compounds that are transported from the blood into the salivary secretion are the major electrolytes; albumin, IgA, immunoglobulin G (IgG), and immunoglobulin M (IgM); and vitamins, drugs, hormones, and water.
There is a good correlation between plasma and salivary levels of a number of
hormones and medications. This correlation forms the basis for proposals to use saliva
collection as a noninvasive means of monitoring hormones and both therapeutic and
illicit drugs. Salivary sampling is currently being tested as a screening method for the
presence of antibodies to human immunodeficiency virus 1. However, such methods
are complicated by the fact that glandular inflammation results in a marked increase
in the number and concentration of serum elements in saliva.
