Some organisms have a remarkable capacity for regenerating tissue. If a fish or salamander suffers heart damage, for instance, their cells are able to divide and successfully repair the injured organ. Imagine if you could do the same.
In the embryo, human heart cells can divide and multiply, allowing the heart to grow and develop. The problem is that, right after birth, cardiomyocytes (heart muscle cells) lose their ability to divide. The same is true for many other human cells, including those of the brain, spinal cord, and pancreas.
