14.1Saliva in Oral Health and Disease

14.1.1Saliva in Dental and Mucosal Defense

Saliva plays a detrimental role in oral health and disease. The ßuid composition and speciÞc components of saliva protect the dental soft and hard tissues by providing an effective set of systems for rinsing, transport of food, bacterial clearance, lubrication of dental surfaces and mucosa, neutralization of acid by buffering actions, maintenance of supersaturation of hydroxyapatite, participation in enamel pellicle formation, and antimicrobial defense [1]. Involving proteins such as mucins, lysozymes, albumin, and anti-proteases, active and passive mechanisms act as a protective shield against desiccation and environmental insult, penetration, ulceration, and potential carcinogens, reviewed in Ref. [2].

Saliva is produced by three paired major glands and numerous minor salivary glands. Over a 24-h period, the average person produces approximately 500Ð1000 mL of whole saliva. Depending on the demand or the current physiological status of the individual, the salivary ßow rates vary considerably [3, 4]. The largest glands are the parotid glands, made up of mainly serous acini and secreting a thin, watery, and amylaserich saliva. The parotid glands contribute to a little less than 50% of the stimulated whole saliva volume. In resting conditions their contribution is much lower. In absence of salivary gland stimulation two-thirds of the resting whole saliva is secreted by the submandibular glands. Although mainly serous, the submandibular glands comprise both serous and mucous acini, but in contrast to the parotid gland, the secretion is more viscous. The smallest of the major glands, the sublingual salivary glands, comprise mainly mucous gland acini and contribute very little to the volume of whole saliva.

The minor salivary glands are mixed glands and largely contain mucous acinar epithelial cells and are located in the oral mucosa and named accordingly; labial, buccal, palatine, lingual, and

glossopharyngeal. In contrast to the low contribution to the volume of whole saliva, the minor glands secrete a large fraction of saliva protein important for lubrication [4].

The protein content and composition of saliva vary depending on from which gland it is secreted. Pure parotid saliva is serous, with approximately the same viscosity as water, whereas submandibular and sublingual saliva is mucous, with a more ÒropyÓ viscosity due to its mucin content [4].

Together with the clearance effect, mucin and the enzymes lactoferrin, lysozyme, and peroxidase have been claimed to play an important role in the non-immune protection of the oral cavity. In addition, agglutinins, histatins, and proline-rich proteins, statherins, and cystatins are among the non-immunologic salivary protein components, reviewed in Refs. [5, 6]. Mucins are the principal organic constituents of mucus, the slimy, viscous material coating all mucosal surfaces and play a role in lubrication, tissue coating, digestion, and microbeÐhost interactions, reviewed in Ref. [7]. Lactoferrin is a protein mainly produced by the interductal cells of serous acini [8]. By binding iron, an important nutrient factor for different oral microorganisms, it displays bacteriostatic/bactericidal, antimycotic, and anti-viral effects, reviewed in Ref. [9]. Lysozyme can exert an anti-microbial function based on its muramidase activity. Lysozyme is a strongly cationic protein, which can activate bacterial autolysins, Òsuicide packagesÓ, that can destroy the bacterial cell walls [5].

Almost all salivary proteins are glycoproteins, i.e., have variable amounts of carbohydrates attached to a protein core. The glycoproteins are often classiÞed according to their cellular origin, mucus and serous glycoproteins, respectively. Glycoproteins are then subclassed by their biological properties, for instance calciumbinding proteins such as statherin and prolinerich proteins, digestive enzymes such as amylase, anti-microbial proteins and peptides such as lysozyme, lactoferrin, and peroxidase systems, and agglutinins [1]. Polymorphism, or occurrence of a protein in multiple forms, is a characteristic feature of glycoproteins, having several