In genetics we have been talking about human adaptations, which are biological adaptations we have made due to our environment. One of the adaptations that we discussed was the adaptation of a larger spleen. This was found in the Sea Nomads of Southeast Asia. These people live at sea on small boats and due to this way of life they get food mostly through diving. So, being able to hold their breath for long periods of time is quite advantageous. Over the years they have developed a genetic mutation that allows them to have a larger spleen. The spleen holds oxygenated red blood cells, so presumably an enlarged spleen – those of the sea nomads, or Bajau people, are about 50 percent larger than the spleens of unrelated, non-diving neighboring groups – injects more blood cells into the circulation and makes more oxygen available for basic body functions during prolonged dives (Sanders, 2018).
What is even more interesting is the physiological mechanism that is believed to be contributing to this adaptation. Levels of the thyroid hormone T4 have been shown to dramatically affect spleen size in mice, suggesting that the observed large spleens of the Bajau result from modulation of thyroid hormone regulation. (Illardo and Nielsen, 2018).
In physiology we learned that there are three types of thyroid hormones including thyroid stimulating hormone (TSH), triiodothyronine (T3), and tetraiodo-thyronine (T4). Thyroid releasing hormone (TRH) is produced in the hypothalamus. When it is released it activates the anterior pituitary gland to release TSH. This then stimulates the thyroid gland to secrete T3 and T4. All three of these thyroid hormones are important for regulating metabolism, growing tissues, ATP consumption, and increasing CV, respiratory, and renal activity (Campisi, 2021). A common medical issue that can arise from elevated thyroid levels is hyperthyroidism, which is when your thyroid gland produces too much of the hormone T4. This can lead to a host of issues including weight loss, irregular heartbeat, fatigue, issues sleeping, and tremors (Pruthi, et al, 2020). While significant research still needs to be done to evaluate what is causing these enlarged spleens, the survival benefit is quite evident. However, as a health care provider how do you treat this population? Do you treat their hyperthyroidism due to all the issues elevated thyroid levels can cause, or do you let it go so that they can have the added benefit of being able to hold their breath longer thus allowing for their survival? Either way, evaluating the biomedical principles of nonmaleficence, beneficence, and autonomy will have to be taken into account.
Campisi, J. 2021, October 6. NS Organization of Endocrine
Ilardo, M., & Nielsen, R. (2018). Human adaptation to extreme environmental conditions. Current opinion in genetics & development, 53, 77–82. https://doi.org/10.1016/j.gde.2018.07.003
Pruthi , S., Acosta, A., Arora, A., Bakkum-Gamez, J., Bakri , S., Baughn , J., Bhatti , M., Gogle , J., Bower , J., Burnett , T., Butler Tobah , Y., Castro , R., Chang-Miller, A., Chebib , F., Creagan , E., Cutsforth-Gregory , J., D'Andre , S., DeSimone, D., David Dingli, … Zhou, M. X. (2020, November 14). Hyperthyroidism (overactive thyroid). Mayo Clinic. Retrieved October 21, 2021, from https://www.mayoclinic.org/diseases-conditions/hyperthyroidism/symptoms-causes/syc-20373659.
Sanders, R. A. 19. (2018, April 20). Enlarged spleen key to diving endurance of 'sea nomads'. Berkeley News. Retrieved October 19, 2021, from https://news.berkeley.edu/2018/04/19/enlarged-spleen-key-to-diving-endurance-of-sea-nomads/.
Hey Jenai! Cool connection between both our genetics and physiology courses. I find it particularly interesting that it is the T4 levels that potentially lead to changes in spleen size. Because the T4 is not the major active form of Thyroid hormone I wonder if this is an additional role (beside modulating responses) of T4.
ReplyDeleteI also find your ethical question interesting. Besides some hypothyroidism are there any major consequences to having a larger spleen? Is it severe enough to cause concern? Is it something like Sickle Cell Anemia. If not, why do you propose we treat something that is not causing an issue? It seems like a case-to-case ethical situation. I also wonder if high levels of T4 could be used to induce better swimmers in competition.