Annalee Coryell

Astronauts have a very unique experience when they are in space, their bodies experience something very unique as well. The lack of gravitational force in space takes a toll on the bodies of astronauts. Because of the usual lack of gravitational force on our bodies that we experience at all times on earth, their bone density decreases at a dramatic rate, up to 1.5% per month. (Stavnichuk et al., 2020)

Our bones and their density compensate and change throughout our lives, with a lack of exercise (mechanical stress) there will be a correlation to the decrease in bone mass.(National Cancer Institute, 2019) Although in space this is even more prevalent because of a complete lack of mechanical stress rather than partial or gradual like we would see with the typical person. This is true for the opposite situation as well, with an increase of mechanical stress, bone density will increase. Our bones go through a cycle of recycling old cells and creating new cells. (The Editors of Encyclopedia Britannica, 2018)

It begins with our osteoclasts, they are located on the surface of the bone, and originate from white blood cells rather than osteogenic cells.(Burns & Smith, 2009) Osteoclasts make up the large majority of our bone cells, accounting for about 90 percent of our bones.(Qin et al., 2020) The osteoclasts bind to the surface of the bone, acidifying the tissue which breaks down the bone cells. Then our osteocytes which are located in the lacune,(Burns & Smith, 2009) essentially decipher how much repair needs to be done and facilitate that. Osteocytes make sure that repair is done to the extent that the body needs and sends signals to the other bone cells to maintain that. The cells help absorb some of the old bone that was already broken down and begin making new bone cells. Creating new cells is also the primary job of the osteoblast. Osteoblasts do not go through mitosis; they do not divide like other cells instead, they are created by the osteogenic cells. The osteogenic cells are found in the growing parts of the bone and are created in the deep layers of the periosteum and in the bone marrow. Once these cells divide and mature they become osteoblasts.(Burns & Smith, 2009) Osteoblasts create new bone cells, they use the needed proteins and other materials needed to create bone matrix, and release it, which then hardens into bone that is able to repeat this process. (Cleveland Clinic, 2023)

When this process is disrupted and mechanical stress is decreased, osteocytes become less active, causing less new bone tissue to be produced and at a slower rate than with a normal level of mechanical stress on the body. With the lack of mechanical stress being chronic or repeated, the bone loses density over time making the loss of density more severe. 

Another Possible reason for decreased bone density after space travel may be a decrease in vitamins consumed from food. (MACK & VOGT, 1971) This was not proven to be a cause for this but we do know that vitamins like calcium which can only come from our food play a huge part in the development of bone. 

I decided to choose this topic because I am very interested in the skeleton and how bones form and how bone marrow is produced and used since it’s an incredibly crucial part of human anatomy. I read about this topic in our textbook, Anatomy and Physiology and was excited to broaden my knowledge of the topic and go beyond what the textbook showed. I covered the course objective “bone development and repair”. 

References

André Santos Campos. (2023). The Moral Entitlements of Future Persons: Expectancies and Prospective Beneficiaries. The Journal of Ethics. https://doi.org/10.1007/s10892-023-09465-2

Burns, P. D., & Smith, S. M. (2009). Human anatomy & physiology laboratory manual, 2E. Bluedoor, Llc.

Cleveland Clinic. (2023, March 27). Osteoblasts & Osteoclasts: Function, Purpose & Anatomy. Cleveland Clinic. https://my.clevelandclinic.org/health/body/24871-osteoblasts-and-osteoclasts

Hadjidakis, D. J., & Androulakis, I. I. (2006). Bone Remodeling. Annals of the New York Academy of Sciences, 1092(1), 385–396. https://doi.org/10.1196/annals.1365.035

National Cancer Institute. (2019). Bone Development & Growth | SEER Training. Cancer.gov. https://training.seer.cancer.gov/anatomy/skeletal/growth.html

Qin, L., Liu, W., Cao, H., & Xiao, G. (2020). Molecular mechanosensors in osteocytes. Bone Research, 8(1). https://doi.org/10.1038/s41413-020-0099-y

Stavnichuk, M., Mikolajewicz, N., Corlett, T., Morris, M., & Komarova, S. V. (2020). A systematic review and meta-analysis of bone loss in space travelers. Npj Microgravity, 6(1). https://doi.org/10.1038/s41526-020-0103-2

The Editors of Encyclopedia Britannica. (2018). Bone remodeling | physiology. In Encyclopædia Britannica. https://www.britannica.com/science/bone-remodeling