In a new study, researchers found when people survive a heart attack, their heart tissue is often irreparably damaged and heart cells cannot regenerate.
This is because the thyroid Hormones halt heart cell regeneration.
The research was conducted by researchers from UC San Francisco.
The thyroid gland produces hormones that can regulate body temperature, metabolic rate and normal heart function in the human body. They can help produce heat to maintain body temperature.
Previous studies have shown that these hormones may be the driving force behind the evolutionary transition from cold- to warm-bloodedness.
Cold-blooded animals were unable to regulate their own body temperature. They are hostage to ever-changing weather conditions and relegated to temperate climates.
On the contrary, warm-blooded mammals were able to live in colder climates and to thrive nocturnally.
In the current study, the team found this evolution advantage came at a big cost.
They found these thyroid hormones also shut off cardiac cell division and hence prevent heart tissue from repairing itself after an injury.
The researchers compared heart cell “ploidy” across 41 different vertebrate species.
Ploidy is the number of copies of each chromosome pair in a cell. It is closely linked to a cell’s ability to divide and replicate.
They found a clear link between ploidy and body temperature.
Cold-blooded animals like fish and reptiles had heart cells that were largely diploid and could respond to cardiac injury via heart cell division.
But warm-blooded mammals had heart cells that were overwhelmingly polyploid, which means these cells rarely divide after cardiac damage.
Their next experiments confirmed that same thyroid hormones responsible for regulating body temperature were also responsible for the diploid-to-polyploid transition and the stop of heart cell division.
The findings suggest that the loss of regenerative potential in the heart cells was a trade-off that makes mammals become warm-blooded.
But when humans can live longer, the loss of heart regeneration becomes a big problem and a fundamental cause of heart disease.
This is the first study connecting thyroid hormones, cardiac development and repair, and the evolution of endothermy.
The lead author of the study is Guo Huang, Ph.D., an investigator at UCSF’s Cardiovascular Research Institute.
The study is published in Science.
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