As a boy, I remember being awed by the ability of some species to regenerate damaged body parts. Cut a starfish in half and both halves can recover to produce two starfish. Cut off a newt’s leg and it can grow a new, fully functional limb in its place. Why can’t people do that? Even more important, why can’t we regenerate tissue to repair damaged organs like our heart or lungs?
Today, researchers are getting closer to understanding why regeneration works in some organisms while not in others. Mammals, it seems, have lost the ability to effectively regenerate many tissues. When mammals lose a limb, the only growth that takes place in the area is low-quality scar tissue. Likewise, when humans suffer organ damage – through a heart attack, for example – the damaged heart cannot repair itself. Instead, scar tissue forms, often preventing the heart from pumping properly. The only human organ to show much regenerative capacity is the liver.
Interestingly, while some amphibians like newts and salamanders are very effective at regenerating limbs, they are not as effective at regenerating tissue to repair damaged internal organs. The striped zebrafish, however, is known to be able to regenerate its spinal cord and retina, as well as its fins. New research has found that the zebrafish can also regenerate heart muscle.
To test this ability, researchers removed 20 per cent of the heart muscle from several zebrafish. For the first few days following surgery, the fish appeared lethargic and uncoordinated. But they improved rapidly. After just one week, they were practically indistinguishable from control fish and after two months, tests indicated that the hearts had fully healed.
Researchers say that after suffering heart damage, zebrafish are able to produce vast numbers of new heart cells, which prevent scar tissue from forming and allow regeneration to take place. Scientists are optimistic that the finding will lead to new ways to treat heart disease. Zebrafish and humans share many genes, including those related to blood, kidneys and eyesight, so zebrafish are widely used as models to help understand the function of human genes.
Studies have shown that mammalian cells appear to maintain the pathways required for tissues to regenerate. MRL, a new strain of mouse bred in the US, has even been able to repair some damage to its heart without scarring. For researchers, this is a clear sign that mammals have the capacity to regenerate organ tissue. The question is, if it does exist in humans, how do we unlock this trait?
Geneticists hope that they will be able to answer some of these questions by comparing the completed mouse and human genomes with the zebrafish genome, which is currently being sequenced, or “mapped” in the UK. Researchers suspect that the secrets to regeneration will be found not in just one gene, but across several.
Another method of repairing damaged human organs that is currently being investigated involves injecting them with human “stem” cells. These rare cells have the capacity to develop into a wide variety of tissues. However, most stems cells are derived from human embryos, which brings up many ethical questions regarding their use. Adult humans still have some stem cells, but these do not seem to have the same potential for regenerative growth as their embryonic counterparts.
Will humans ever be able to re-grow damaged limbs or organs like newts and zebrafish? It’s impossible to say. But the potential for regeneration appears to reside in our genes. Considering that heart disease kills tens of thousands of Canadians every year, that’s a potential we would do well to unlock.
– David Suzuki