|Year : 2012 | Volume
| Issue : 2 | Page : 72-76
Saliva and dental practice
Aiswaryalekshmy Sreelatha Umeswaran Nair, Rooban Thavarajah, Kannan Ranganathan
Department of Oral and Maxillofacial Pathology, Ragas Dental College and Hospital, Uthandi, Chennai, India
|Date of Web Publication||11-Jul-2012|
Professor, Department of Oral and Maxillofacial Pathology, Ragas Dental College and Hospital, Uthandi, Chennai-600119
Human saliva plays an important role in the health of the oral cavity and of the body as a whole. It is a complex fluid that is actively secreted by the major and minor salivary glands. Its composition shows variations that reflect the physiology and the systemic health of the individual. It has the potential to be used for early detection of disease and assessment of its progression and treatment outcome. As a diagnostic tool, saliva has the unique advantage of being readily-available, and its collection is non-invasive. The field of salivary diagnostics and research is gaining importance and is becoming a major area of focus for researchers. Discovery of salivary biomarkers and its validation had broadened the use of salivary diagnostics from assessment of dental caries to the diagnosis of cardiac diseases and malignancies remote from the oral cavity. This article intends to give an overview of this exciting area of salivary research.
Keywords: Oral diagnostics, saliva, salivary pH, systemic diseases
|How to cite this article:|
Nair AU, Thavarajah R, Ranganathan K. Saliva and dental practice. J Dr NTR Univ Health Sci 2012;1:72-6
| Background|| |
Saliva is a complex biological fluid, which is necessary for the maintenance of oral as well as general health state. Human saliva provides a window to the physiological and pathological state of the body as a whole. Salivary diagnostics is an emerging field that has tremendous potential in clinical applications because its collection is non-invasive and it contains a wide spectrum of analytes, which can serve as biomarkers for assessment of oral and systemic health.
| Overview of Saliva Physiology and Composition|| |
The development of human salivary glands begins early (6-8 weeks) in the intrauterine life from oral ectoderm and the ectomesenchyme.  Saliva is secreted from 3 pairs of major salivary glands - parotid gland, submandibular gland, sublingual gland and the numerous minor salivary glands present in the oral mucosa (labial, buccal, lingual and palatal). The salivary glands are composed of 2 types of specialized epithelial cells: The acinar and ductal cells. Salivary secretions are classified as serous, mucous, or mixed. Serous secretions contain more water than mucous. Parotid saliva is serous, submandibular saliva is mixed but more serous than mucous, and sublingual saliva is mixed, but mostly mucous. Mostly, the minor salivary glands produce a mucous salivary secretion.
Saliva is of 2 types: Unstimulated (resting) saliva and stimulated saliva. In unstimulated (resting) state the parotid, submandibular, sublingual and minor salivary glands contributes approximately 25%, 60%, 7%-8%, and 7%-8% respectively to the whole saliva volume.  Stimulation affects the quantity of saliva, the concentrations of some constituents, and the pH of the fluid. Normal whole saliva secretory rates vary between 800 and 1,500 ml/day with a pH in the range of 6.0-7.0.  Stimulation affects the quantity of saliva, the concentrations of some constituents, and the pH of the fluid.
Salivary flow rate varies among individuals, and it is influenced by environmental factors. It is greater in standing position than in sitting. In cool weather, it is greater when compared to hot weather. In addition, saliva is subjected to a circadian rhythm, with the highest flow in mid afternoon and the lowest around 4:00 AM.  Circadian rhythms, physical activity, and health status of the individual also influence physiology and flow rate of oral fluid. 
Xerostomia or dry mouth is the clinical awareness of decreased salivary secretion, and it may even occur without a low salivary flow rate.  Hyposialia refers to a salivary flow rate less than 0.16 ml/min. It may develop in many local and systemic conditions including Sjögren's syndrome, diabetes mellitus, drug exposures, radiation to the head or neck. 
The buffering capacity of saliva is also an important factor in dental remineralization. It depends on bicarbonate concentration and salivary flow rate as any factor decreasing salivary flow rate tends to decrease buffer capacity of saliva and to increase the risk of caries development. 
Besides water, saliva contains proteins, post-translationally-modified proteins (glycoproteins, phosphoproteins), peptides, lipids, minerals, and other small compounds. Upon release of glandular secretions into the oral cavity, the fluid is mixed with a variety of exocrine, non-exocrine, cellular, and exogeneous components to ultimately form whole saliva (WS) [Figure 1]. Thus, whole saliva is a complex fluid, which consists of salivary secretions from different salivary glands, sloughed oral epithelial cells, nasopharyngeal discharge, food debris, gingival crevicular fluid, bacteria, and their products. 
| Functions of Salivary Components|| |
Saliva has several types of functions that are of profound importance for the oral health [Table 1]. 
| Salivary Diagnostics|| |
Salivary diagnostics is a dynamic and emerging field in the diagnosis of oral and systemic diseases. Saliva reflects the physiologic state of the body, including emotional, endocrinal, nutritional, and metabolic variations. The achievements of high-throughput approaches such as proteomics including free-flow electrophoresis coupled with linear ion-trap tandem mass spectrometry, multidimensional separation platform based on nano-reversed phase liquid chromatography and capillary isoelectric focusing, two dimensional gel electrophoresis (2-DE) coupled with matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) or liquid chromatography-mass spectrometry (LC-MS/MS), iTRAQ (isobaric tag for relative and absolute quantitation) and label-free quantitation, microarray and microfluidics afforded by modern diagnostic techniques allow for disease-specific salivary biomarker discovery and establishment of multiplex, rapid, and miniaturized analytical salivary assays.
Saliva meets the demands for an inexpensive, non-invasive, and easy-to-use diagnostic platform. The advantages of easy collection, storage, shipping, and voluminous sampling, make saliva a better diagnostic biofluid than serum or urine. It is also easier to handle saliva during diagnostic procedures than blood because it does not clot, and this reduces the number of manipulations required. It is safer when compared to serum for health care professionals as serum is more likely to expose operators to blood-borne diseases. The non-invasive collection of saliva could reduce anxiety and discomfort for the patients and increase their willingness to undergo investigations. This will greatly increase the opportunity for early diagnosis of diseases and to monitor general health. 
Salivary parameters such as salivary flow rate, salivary pH, salivary viscosity, and salivary buffering capacity are found to be lower in subjects with dental caries.  Salivary flow rate is the most important parameter since the cariostatic activity or efficacy of all other salivary parameters depends on the flow rate. Flow rate has no linear association with dental caries, but there is an individual "threshold" limit, which is decisive for enhanced caries activity. A longitudinal follow-up of the flow rate in a regular basis is of higher clinical value than only a single cross-sectional measurement.  Salivary buffer effect has only a weak negative association with caries activity, and this effect is of greater clinical significance on a population level. The buffering effect of saliva is limited in dental caries as the decisive processes in caries attack occur within or under the dental plaque. Salivary tests like the Caries Assessment and Risk Evaluation (CARE) test, colorimetric test for assessment of the buffer potential of saliva, Dentocult® for the search for Streptococcus mutans s for lactobacilli are available.
Many studies are done in search of biomarkers for periodontal diseases including salivary levels of Matrx metallo proteinase-8,  Interleukin-1β.  Tumor necrosis factor-α; Aspartate aminotransferase; Elastase, Collagen telopeptides; Osteocalcin; Proteoglycans; Fibronection fragments.  Works are under progress for identification and validation of panels of protein biomarkers, with particular emphasis on identifying groups of proteins that have relevance to all stages of periodontal disease progression.
Oral cancer and oral potentially malignant disorders
Discovery and validation of differentially expressed proteins in saliva from patients with oral squamous cell carcinoma that could serve as potential biomarkers have thrown light to early detection of oral cancer. Proteomic analysis of saliva has revealed many different biomarkers in oral cancer like p53, M2bp, profilin, CD59, MRP14, catalase, S100, hematopoietic lineage cell specific protein.  Transcriptomic analysis of saliva also revealed biomarkers DUSP1, H3F3A, IL1 β , IL8, OAZ1, S100, and SAT.  Significant increases (of 400%) in salivary concentrations of Cyfra 21-1, tissue polypeptide antigen, and CA125 were also shown.  Saliva miRNAs were also studied identifying the presence of dozens of miRNAs in saliva of oral cancer patients. 
Malignant lesions remote from the oral cavity
Investigators have identified 57 principal metabolites in saliva that can be used to predict the probability of being affected by oral, breast, and pancreatic cancer. Quantitative information for these metabolites and their combinations enable us to predict disease susceptibility. These metabolites could serve as promising biomarkers for medical screening of malignant lesions. , Transcriptomic and proteomic signatures in saliva were also identified, which can serve as biomarkers for the non-invasive detection of breast cancer.
Composition of saliva is influenced directly or indirectly by most of the systemic diseases. Many researches in this field have enabled us to bridge oral health research with systemic disease diagnosis. Early detection, diagnosis, and assessment of prognosis of systemic diseases could be possible by monitoring the composition of saliva. 
| Diabetes|| |
Salivary glucose concentration is a potentially useful non-invasive tool to monitor glycemic control in diabetic patients.  Analysis of saliva in diabetes patients revealed 65 proteins, which are relatively abundant in the saliva of diabetics compared to controls. A subset of these proteins was found to be differentially abundant in patients with prediabetes (individuals with impaired glucose tolerance and/or impaired fasting glucose). Independent validation of proteins, carried out through the method of immunodetection, confirmed the differential expression of the subset of biomarkers in diabetes patients. Prediabetic sample analysis also showed a relative increase in protein abundance with disease progression.  An overexpression of chromogranin A in saliva of diabetic patients was identified. Chromogranin A, a circulating biomarker for epithelial tumors, is overexpressed in saliva of type 2 diabetic patients. 
| Myocardial Infarction|| |
Biomarkers like C-reactive protein (CRP), soluble CD40 ligand (sCD40L), myeloperoxidase, myoglobin, tumor necrosis factor, soluble intercellular adhesion molecule-1, and matrix metalloproteinase 9 have been studied and proven in saliva to be significantly altered in subjects with myocardial infarction as compared to normal subjects. 
| Sjögren's Syndrome|| |
Proteomic analysis and independent validation of salivary components have yielded 27 significantly up-regulated mRNAs in Sjögren's syndrome patients. Also, 19 of the 27 over-expressed gene transcripts were known products of an immune response to pathogenesis. In samples taken from Sjögren's syndrome patients, a set of 16 whole saliva peptides and 40 salivary proteins were also identified that were at significantly different levels than those from the healthy volunteers.  Some of these are Salivary α-amylase, Carbonic anhydrase, Caspase 14, β 2 -microglobulin, Calgranulin B, and Actin. Salivary anti-Ro60 and anti-Ro52 antibody profiles have been used to diagnose Sjφgren's syndrome. 
| Infectious Diseases|| |
Helicobacter pylori that has been associated with gastric inflammation exists in saliva, and the oral-oral route may be an important means of transmission of this infection.  Researchers have proved that diagnosis of infection with the human immunodeficiency virus (HIV) based on specific antibody in saliva is equivalent to serum in accuracy, and therefore, is applicable for both clinical use and epidemiological surveillance (Antibody to HIV in whole saliva of infected individuals, which was detected by ELISA and Western blot assay, correlated with serum antibody levels). Detection of antibody to HIV in saliva showed sensitivity and specificity between 95% and 100% when compared with serum. Point of care devices like OraQuick® and Orasure® are commercially available. 
| Drug Monitoring|| |
Saliva has been proposed for the monitoring of systemic levels of drugs similar to other body fluids (i.e., serum, urine, and sweat). Saliva may be used for monitoring patient compliance with psychiatric medications, anti-cancer drugs, illicit drug use, monitoring of therapeutic drugs, ethanol, recreational drugs, and Cotinine (cigarette smoking).  Oral- AQ Oral Fluid Drugs of Abuse Rapid Test® is a commercially available test for drug abuse.
| Hormone Levels|| |
Analytes in saliva can be used as part of the evaluation of endocrine function. Levels of hormones like Cortisol, Aldosterone, Testosterone, Dihydroepiandrosteron, Estradiol, Progesterone, Insulin, Protein hormones gonadotrophins, prolactin, thyrotropin etc. could be analyzed in saliva for monitoring of health and diseased states.  Salivary cortisol levels are being used to assess stress levels.
| Role of Saliva in Forensic Sciences|| |
Saliva plays a major role in forensic sciences. Some of the areas are evaluation of chemical markers (thiocynate ion, amylase), serological markers (blood group substances, macro molecular glycoproteins) for individual identification and of substance abuse, sex determination, and DNA analysis.
| Conclusion|| |
Saliva and whole mouth fluid is a perfect medium to be explored for health and disease surveillance.  Use of novel approaches that include metabolomics, genomics, proteomics, and bioinformatics for assessment of biomarkers in oral as well as systemic diseases have made salivary research a rapidly-advancing field. Much work needs to be done before turning saliva diagnostics into a clinical and commercial reality. Identification and validation of robust and reliable salivary biomarkers and introduction of point-of-care testing and real-time disease surveillance could propel saliva diagnostics to reality and could dramatically change the clinical practice.
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