**1. Introduction**

The oral cavity is the body's portal for the intake and initial processing of food and liquids, functioning in taste, mastication, solubilization and digestion of nutrients. Additionally, the oral cavity plays a role in respiration and speech as well as in innate and adaptive immunity. The components of the oral cavity include the teeth, consisting of three types of mineralized tissue, and their supporting structures, the bones of the maxilla and mandible and alveolar processes, and the periodontal ligament. The oral mucosa, which covers the alveolar processes, tongue, palate, cheeks, inside of the lips and floor of the mouth, consists of keratinized and non-keratinized epithelium overlying the connective tissues of the lamina propria and submucosa.

Located extraorally are the major salivary glands: the parotid, the submandibular and the sublingual, which convey their product, saliva, to the oral cavity *via* long ducts. Saliva, consisting of water, electrolytes and a large number of proteins, glycoproteins and small organic substances, creates and regulates the environment of the oral cavity. Changes in composition and rate of secretion of saliva have significant effects on oral tissues as well as on general health. In addition to the major glands, numerous small minor salivary glands are present in the subepithelial connective tissue throughout much of the oral cavity.

Among the organic constituents of saliva are digestive enzymes, calcium binding proteins, a variety of growth factors and regulatory molecules, antimicrobial components and immunoglobulins, and mucins that lubricate and moisten the oral tissues [1]. Other protective components of saliva include those involved in buffering and neutralizing acids produced by oral microorganisms and ingested with food and drink. Most are produced and secreted by the cells of the salivary glands, nevertheless a number of other proteins derived from other cells, tissues and organs also find their way into saliva. The presence of these substances and the ease of, and the noninvasive means of, collecting saliva have led to a great deal of interest for its use as a diagnostic fluid. Consequently, significant progress has been made in using saliva to detect oral and other cancers, several oral and systemic diseases, and to assess physiological and environmental stressors [2].

Our early studies of rats flown on Spacelab-3 [3, 4] indicated that exposure to microgravity resulted in specific changes in salivary gland structure and biochemistry. Accordingly, the premise for our recent studies of mice flown on the US space shuttles and the Russian Bion-M1 biosatellite, was that changes in the expression of salivary gland secretory proteins appear to occur in microgravity, and that the pattern of changes detectable in saliva could be employed to assess important aspects of the physiological status of astronauts.

In addition to studying salivary gland tissues, we examined the effects of microgravity on the mandibles and teeth of mice flown in space. Numerous studies have been carried out documenting the effects of spaceflight on bones of the weightbearing skeleton, especially the bones of the lower limbs and vertebrae (see reviews in [5–7]). However, only a few studies have focused on non-weight-bearing bones, such as the mandible and cranial bones, and teeth [8, 9]. Teeth, once formed, are relatively stable structures, but can be affected by changes in the oral environment. An understanding of the effects of microgravity on tooth development, which can be studied using the continuously erupting rodent incisor, is important especially in view of future possibilities of long-term space journeys and colonization of other planets.
