**5. Effects of spaceflight on other oral tissues**

There are few reported studies of the effects of spaceflight on oral tissues other than bone and teeth. Histologic examination of periodontal tissues and oral mucosa revealed no differences between mice flown on Apollo 17 for 12.5 days and ground control mice [37]. Histologic studies of the lingual mucosa of rhesus monkeys after 30 days of simulated weightlessness showed no differences from control monkeys [98]. The masseter muscles of mice flown for 13 days on STS-135 showed no change in mass, fiber size distribution, signaling pathways, or the expression of genes associated with muscle atrophy compared to ground control mice [99]. Power output was decreased about 40% in the masseter muscles of flight mice, but maximal shortening velocity was not affected. In contrast, tibialis anterior muscles of these flight mice underwent atrophy, with loss of mass, changes in signaling pathways and gene expression, and loss of strength. In a separate experiment, mice fed a liquid diet for 2 weeks showed a decrease in masseter muscle fiber size of more than 40%. These results suggest that the load imposed by chewing hard food protected the masseter muscles from the microgravity-induced atrophy occurring in appendicular muscles, but was insufficient to preserve muscle power.
