When people have heart attacks, oxygen is cut off from the cells of the heart, called cardiomyocytes, and those cells consequently die. If this happened in the liver, the immune system would remove the dead cells and the surrounding cells would reproduce to replace them. But things are different in the heart. The immune system still removes the dead cells, but the cardiomyocytes don’t reproduce enough to replace them. Instead, a scar is formed which means that the heart can’t pump as well. Ultimately, if the heart’s function, pumping blood to the rest of the body, is affected too much by the loss of cardiomyocytes, it can lead to heart failure. Scientists are working hard to find out why cardiomyocytes will not replicate, what is different about the very small population of cardiomyocytes that do replicate and how can we enhance this replication when someone has a heart attack.
A normal cell in your body has two copies of your DNA. These cells are called diploid cells. If a cell begins to replicate and stops halfway through, meaning the DNA was duplicated but the cell did not split into two new cells, then that cell, which now has four copies of DNA, is said to be polyploid. In the heart, almost all cells are diploid at birth, but overtime the majority of cardiomyocytes become polyploid. In the mouse heart, all cardiomyocytes are diploid at birth, and at this time the heart is capable of complete regeneration. This means if a newborn mouse has a heart attack, the cardiomyocytes reproduce enough to replace the lost cells. Once the mouse is 7 days old, almost all of the cardiomyocytes have become polyploid and the heart is no longer capable of regenerating. This has led many scientists to believe that this transition to polyploidy, which coincides with the transition from being able to regenerate to not being able to regenerate, is why the cardiomyocytes are incapable of reproducing. Interestingly, zebrafish (check out our model organisms post) are able to regenerate their heart throughout their life and the cardiomyocytes in zebrafish hearts remain completely diploid. So, scientists are interested in finding out if the small population of diploid cardiomyocytes remaining in the human adult heart are capable of reproducing.
Scientists have not found any differences between diploid and polyploid cardiomyocytes beyond the number of DNA copies that they have. They function the same within the heart and express the same genes.
Once the type of cardiomyocyte that can regenerate has been identified, then it can be studied in more detail to find out what gives it the ability to reproduce. In the ideal, simplified world, a drug could then be made to enhance this difference whenever someone has a heart attack – effectively signaling to the cells to reproduce and replace the damaged part. Then, little or no scar would form on the heart and function would be retained. We are far off from this, but research as a whole is taking baby steps towards a bigger picture.
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Written by Annah
Illustrated by Rhea