Space flight into low Earth orbit and beyond sounds appealing, but there are obvious caveats. At least, that is the immediate impression that the casual observer gets when thinking of the Challenger or Columbia tragedies. However, perusing the space medicine literature from US/Soviet Sky Lab missions to the experiments at the International Space Station one gets a completely different impression of the depth of bio-medical research in space. For an extended trip past the confines of our planet it is painfully aware to a select few medical scientists, astronauts and technicians that the first trips to Mars will need to be extensively performed upon test subjects (cellular and living). Here are some of the reasons why I believe this to be true:
- low gravity conditions affect the immune response in human subjects which in turn weakens the body’s defense to radiation at lower levels
- cataract(s) formation is known to occur
- prolonged exposure to ionizing radiation is known to induce cancer (i.e. tumors, leukemia)
- cellular death (apoptosis) occurs more readily in high radiation environments
- loss of bone density (i.e. calcium depletion)
- kidney stone formation
- heart rhythm irregularities
- muscle atrophy
- damage to the central nervous system and gut
Thus, I will survey one organ that is known to be affected during a long journey away from our planet: the thyroid gland. (I will give a list of references at the end of the post that are freely available on internet on various aspects of spaceflight and the human body.) The implications of just this one gland may seem overwhelming to some, but it should drive home what the current state of technology may need to address if the “average joe or joan” wants to leave Earth for an extended vacation.
The thyroid gland is located in the neck under the voice box (see Fig. 1) and the thyroid’s function is regulated by the pituitary gland (see Fig. 2) located at the base of the brain. The thyroid’s function is fairly complicated and multi-faceted. [The thyroid produces three hormones: T3 (Triiodothryonine), T4 (thyroxine), and Calcitonin (which I will call C). T3 and T4 are primarily responsible for energy metabolism—growth, nerve responsiveness, and internal body temperature. Hormone C participates in calcium and bone metabolism. The thyroid/pituitary mechanism is (also complicated) to stimulate the thyroid to produce T3 and T4.]
Before I speak of the effects of space flight upon the thyroid, I will add the following illustration of how the thyroid system works for further simplification(?):
- Hypothalamus sends a message to the pituitary gland for thyroid to act
- Thyroid releases T3 and T4 into the body to increase metabolism (the catecholamine effect pertains to mood and the central nervous system)
- Once the body has received and acted upon the T3 and T4, the thyroid sends negative feedback to the brain to stop the pituitary and hypothalamus actions upon the thyroid
What are the known effects of Space Flight upon the thyroid gland?
Hypothyroidism is documented in humans (and animals) and decreased hormonal levels of T3 and T4 (with an increase in TSH–see fig. 3 above) are symptomatic of the condition. However, when the astronauts returned to Earth, thyroid capacity returned to pre-flight levels. This does beg the question of what would happen during an extended (i.e. 3 year trip to Mars) stay in the Solar System? [It is, also, well documented that the thyroid (like other bodily organs) are adversely affected by the low gravity conditions.] I am unaware of the current state of space medicine in regards to extended trips in the solar system, but that means that I could not find the pertinent references in a 48 hour period of search 😦 . [Perhaps, I didn’t look in the proper areas?]
However, in an upcoming 2013 journal article in Astrobiology, an article addresses the effects of low-level ultraviolet radiation upon rat thyroid cells. The nine authors studied the effects of UV radiation (without microgravity, e.g. in their laboratory) upon the cellular level of thyroid expression. So, in a nutshell, the investigators attempted to simulate the effects of low-level radiation upon thyroid cells (and cellular function) in the hopes of learning how thyroid DNA was affected.
Their results showed that low-level radiation (a level that did not kill thyroid cells) slowed cell growth and affected gene expression (DNA) as well. The authors did not investigate how the DNA was chemically affected or in what manner. They did offer a bio-chemical explanation (or hypothesis for further investigation) that may be in line with “free radical damage” or “photo-chemical reactions upon DNA, itself.”
And apparently, the results dovetail with a previous study done on the ISS where thyroid function was measured on cell lines. The previous study indicated that thyroid cell function diminished in low-Earth orbit, but no DNA studies were performed at the time.
Perhaps one may also infer that a lack of trips from the Earth from space faring nations and entities could mean that our technology is not advanced to the level of sending humans beyond low-Earth orbit for longer than 1 week.
Low gravity and radiation have deleterious effects upon the thyroid gland. (as maybe inferred from the general effects of microgravity/ionizing radiation) And, the effect of radiation may be analogously viewed as if one were being dosed with radiation; low-levels of radiation produce free radicals in the body and induce unwanted bio-chemical reactions in one’s DNA .
We are, in many ways, still ill-informed on the complete effects of low-gravity and space radiation upon the human body. Despite our celebrations of 50 years in space, we have viewed certain aspects of space flight unrealistically. Sadly however, a majority of the current populace will never venture past the confines of their current home. Perhaps this is the true state of our technology advancement?
Journal References for Thyroid Function
Albi, E. et al (2010) Thyroid cell growth: sphingomyelin metabolism as non-invasive marker for cell damage acquired during spaceflight. Astrobiology 10:811-820
Baldini, E. et al (2013) Effects of Ultraviolet Radiation on FRTL-5 Cell Growth and Thyroid-Specific Gene Expression. Astrobiology. 13 (doi: 10.1089/ast.2013.0972)
Freely Available Articles From Google Scholar Search:
Allen, D. L., Bandstra, E. R., Harrison, B. C., Thorng, S., Stodieck, L. S., Kostenuik, P. J., Morony, S., et al. (2009). Effects of spaceflight on murine skeletal muscle gene expression. Journal of applied physiology (Bethesda, Md. : 1985), 106(2), 582–95. doi:10.1152/japplphysiol.90780.2008
Higashibata, A., Szewczyk, N. J., Conley, C. a, Imamizo-Sato, M., Higashitani, A., & Ishioka, N. (2006). Decreased expression of myogenic transcription factors and myosin heavy chains in Caenorhabditis elegans muscles developed during spaceflight. The Journal of experimental biology, 209(Pt 16), 3209–18. doi:10.1242/jeb.02365
Layne, C. S., McDonald, P. V, & Bloomberg, J. J. (1997). Neuromuscular activation patterns during treadmill walking after space flight. Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale, 113(1), 104–16. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/9028779
Lalani, R., Bhasin, S., Byhower, F., Tarnuzzer, R., Grant, M., Shen, R., Asa, S., et al. (2000). Myostatin and insulin-like growth factor-I and -II expression in the muscle of rats exposed to the microgravity environment of the NeuroLab space shuttle flight. The Journal of endocrinology, 167(3), 417–28. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11115768
Banerjee, S., Evanson, J., Harris, E., Lowe, S. L., Thomasson, K. a, & Porter, J. E. (2006). Identification of specific calcitonin-like receptor residues important for calcitonin gene-related peptide high affinity binding. BMC pharmacology, 6, 9. doi:10.1186/1471-2210-6-9
Chapes, S. K., Mastro, a M., Sonnenfeld, G., & Berry, W. D. (1993). Antiorthostatic suspension as a model for the effects of spaceflight on the immune system. Journal of leukocyte biology, 54(3), 227–35. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/8371052
Young, L. R. (1996). Effects of orbital space flight on vestibular reflexes and perception. Acta astronautica, 36(8-12), 409–13. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11540971
Albi, E., Curcio, F., Spelat, R., Lazzarini, A., Lazzarini, R., Cataldi, S., Loreti, E., et al. (2012). Loss of parafollicular cells during gravitational changes (microgravity, hypergravity) and the secret effect of pleiotrophin. PloS one, 7(12), e48518. doi:10.1371/journal.pone.0048518
Martín-Lacave, I., Borrero, M. J., Utrilla, J. C., Fernández-Santos, J. M., De Miguel, M., Morillo, J., Guerrero, J. M., et al. (2009). C cells evolve at the same rhythm as follicular cells when thyroidal status changes in rats. Journal of anatomy, 214(3), 301–9. doi:10.1111/j.1469-7580.2008.01044.x
Miller, P. B., Hartman, B. O., Johnson, R. L., & Lamb, L. E. (1964). Modification of the Effects of Two Weeks of Bed Rest Upon Circulatory Functions in Man. Aerospace medicine, 35, 931–9. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/14198654
Lewis, M. L., Reynolds, J. L., Cubano, L. a, Hatton, J. P., Lawless, B. D., & Piepmeier, E. H. (1998). Spaceflight alters microtubules and increases apoptosis in human lymphocytes (Jurkat). FASEB journal : official publication of the Federation of American Societies for Experimental Biology, 12(11), 1007–18. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/9707173
Allebban, Z., Lange, D., Congdon, C., Ichiki, T., Gibson, A., & Jones, B. (1994). Effects of spaceflight on the number eukocytes and lymphocyte subsets of rat peripheral blood, 55(February), 209–213.
Smith, S. M. (2002). Red blood cell and iron metabolism during space flight. Nutrition (Burbank, Los Angeles County, Calif.), 18(10), 864–6. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/12361780
Watenpaugh, D. E. (2001). Fluid volume control during short-term space flight and implications for human performance. The Journal of experimental biology, 204(Pt 18), 3209–15. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11581336