**6. Conclusions**

Oral tissues, particularly the salivary glands, mandible and teeth, are affected by spaceflight. The 3 major salivary glands respond differently to the effects of microgravity and the spaceflight habitats. The PG exhibits a number of morphological changes that are also seen following restriction of food, feeding of a liquid diet, and experimental diabetes. The expression of some secretory proteins is increased, whereas others are decreased; for some proteins the length of microgravity exposure may be important. A number of genes also exhibit increased or decreased expression. The response of the SMG to spaceflight appears to differ between males and females, with male glands exhibiting increases in expression of several secretory proteins and female glands remaining unaltered. The SLGs of female mice flown on the space shuttles showed a few changes in secretory protein expression. Interestingly, the expression of the stress protein PKA-RII and its gene was altered in the shorter space shuttle flights, but not the longer Bion-M1 flight. This may indicate an adaptation to the spaceflight environment, or a relatively rapid return to baseline during the longer period between landing and sample collection for the Bion-M1 mice. The ease of saliva collection makes it a potential alternative to blood for monitoring physiological responses to spaceflight and the health of astronauts and cosmonauts. Recent studies have documented substantial differences in male and female salivary gland gene expression in both mice [100] and humans [101]; therefore, additional, more comprehensive studies of the effects of spaceflight on the saliva proteome are needed.

Not considered in our studies, but important for overall oral health, are the minor salivary glands. Although they produce less than 10% of total saliva volume, these glands secrete continuously, providing mucins and antimicrobial substances that lubricate, moisten and protect the oral tissues, especially during periods when the major glands are inactive, such as during sleep. Microgravity induced alterations in their secretions could have significant effects on the teeth and oral mucosa.

The effects of spaceflight on the mandible, observed in our studies, are consistent with the observations reported by others for non-weight bearing skeletal elements. Changes in BV and bone architecture are minimal in the mandible compared to long bones of the lower extremity and vertebrae. The decrease in BV seen in the Bion-M1 flight and habitat control mice most likely was due to the soft paste diet consumed by these mice. Microgravity probably also has an effect, considering the increased proportion of sclerostin-positive osteocytes and number of osteoclasts in the mandible seen in the flight mice. Similarly, the significant changes observed

**71**

**Author details**

Maija I. Mednieks and Arthur R. Hand\*

provided the original work is properly cited.

UConn Health, Farmington, Connecticut, USA

The authors have no conflicts of interest.

\*Address all correspondence to: hand@uchc.edu

*Oral Tissue Responses to Travel in Space DOI: http://dx.doi.org/10.5772/intechopen.86728*

**Acknowledgements**

[ARH]) and UConn Health.

**Conflict of interest**

in incisor tooth volume and mineralization appear to be related to the soft diet and slowing of the eruption rate. Changes in tooth protein expression may be a combined result of microgravity, diet and the habitat environment. The change from "normal" conditions, the confined space, and different diet, must also be consid-

Many of the reported animal and human studies, including ours, would be improved with a larger number of subjects, increasing statistical power and strengthening observed trends. Nevertheless, many significant changes have been documented, and should be considered in planning future animal studies and especially long-term human spaceflight. The recent NASA Twins Study [47] emphasizes the need for comprehensive studies of long-term exposure to the spaceflight environment. Advances in spacecraft and experimental design will resolve current

The authors' studies were supported by NASA (grant number NNX09AP13G [MIM]), the Connecticut Space Grant College Consortium (grant numbers P-536 and P-777 [ARH]), NIH (Shared Instrumentation Grant number 1S10RR024742

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

ered, and adequately controlled for in experimental studies.

limitations and improve outcomes of future travel in space.

## *Oral Tissue Responses to Travel in Space DOI: http://dx.doi.org/10.5772/intechopen.86728*

*Beyond LEO - Human Health Issues for Deep Space Exploration*

dicular muscles, but was insufficient to preserve muscle power.

**6. Conclusions**

the saliva proteome are needed.

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 appen-

Oral tissues, particularly the salivary glands, mandible and teeth, are affected by spaceflight. The 3 major salivary glands respond differently to the effects of microgravity and the spaceflight habitats. The PG exhibits a number of morphological changes that are also seen following restriction of food, feeding of a liquid diet, and experimental diabetes. The expression of some secretory proteins is increased, whereas others are decreased; for some proteins the length of microgravity exposure may be important. A number of genes also exhibit increased or decreased expression. The response of the SMG to spaceflight appears to differ between males and females, with male glands exhibiting increases in expression of several secretory proteins and female glands remaining unaltered. The SLGs of female mice flown on the space shuttles showed a few changes in secretory protein expression. Interestingly, the expression of the stress protein PKA-RII and its gene was altered in the shorter space shuttle flights, but not the longer Bion-M1 flight. This may indicate an adaptation to the spaceflight environment, or a relatively rapid return to baseline during the longer period between landing and sample collection for the Bion-M1 mice. The ease of saliva collection makes it a potential alternative to blood for monitoring physiological responses to spaceflight and the health of astronauts and cosmonauts. Recent studies have documented substantial differences in male and female salivary gland gene expression in both mice [100] and humans [101]; therefore, additional, more comprehensive studies of the effects of spaceflight on

Not considered in our studies, but important for overall oral health, are the minor salivary glands. Although they produce less than 10% of total saliva volume, these glands secrete continuously, providing mucins and antimicrobial substances that lubricate, moisten and protect the oral tissues, especially during periods when the major glands are inactive, such as during sleep. Microgravity induced alterations in their secretions could have significant effects on the teeth and oral mucosa. The effects of spaceflight on the mandible, observed in our studies, are consistent with the observations reported by others for non-weight bearing skeletal elements. Changes in BV and bone architecture are minimal in the mandible compared to long bones of the lower extremity and vertebrae. The decrease in BV seen in the Bion-M1 flight and habitat control mice most likely was due to the soft paste diet consumed by these mice. Microgravity probably also has an effect, considering the increased proportion of sclerostin-positive osteocytes and number of osteoclasts in the mandible seen in the flight mice. Similarly, the significant changes observed

**70**

in incisor tooth volume and mineralization appear to be related to the soft diet and slowing of the eruption rate. Changes in tooth protein expression may be a combined result of microgravity, diet and the habitat environment. The change from "normal" conditions, the confined space, and different diet, must also be considered, and adequately controlled for in experimental studies.

Many of the reported animal and human studies, including ours, would be improved with a larger number of subjects, increasing statistical power and strengthening observed trends. Nevertheless, many significant changes have been documented, and should be considered in planning future animal studies and especially long-term human spaceflight. The recent NASA Twins Study [47] emphasizes the need for comprehensive studies of long-term exposure to the spaceflight environment. Advances in spacecraft and experimental design will resolve current limitations and improve outcomes of future travel in space.
