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I know that it is possible for certain animals to regrow limbs, but is it possible for humans to regrow limbs sometime in the future? Why are humans not able to regrow limbs but other animals can?
Question Date: 2019-11-06
Answer 1:

That’s a great question. A lot of scientists have researched it, but haven’t yet figured out how exactly we could help people re-grow limbs. We do have some idea of what the process looks like in other animals. Check out this short video clip of regeneration in a salamander (an amphibian).

Developmental biologists study how one fertilized egg becomes trillions of specialized cells that make up a human (or whatever species they study). Part of the answer for vertebrates like us is that cells communicate with each other. Important information about time, location, and what others cells are doing allows them to “cooperate” in a way so that you have a hand where a hand belongs and not a foot, elbow, or liver.

Development is an extremely complex process of cell division and specialization. All cells start from that one fertilized egg. Early on, all the cells are the same and could become any type of cell. They are stem cells and could become skin cells, or liver cells, or muscle cells, for example. If the whole embryo is split in two, two healthy identical twins would form. After several divisions, though, cells begin to specialize. This is called “differentiation” because the cells become permanently different.

You may have a picture in your textbook labelled “animal cell.” I bet it’s round. But one drawing to represent all animal cells is like one animal drawing to represent all animals (coral, worms, clams, eagles, cows, beetles, etc.). In reality, muscle cells liver, cells, and skin cells look very different. Their structure is what allows them to function in very different ways. So you have an arm with cells differentiated into muscle, bone, nerves, etc. If you lose your arm, you need to have all the cell types assemble a new arm. That’s not easy.

All cells in an individual (except for gametes and mature red blood cells) have the same DNA, but each cell type only uses some of the genes. The rest are “switched off.” The cells that become bone cells are genetically the same as the cells that became nerve cells, but their structure is very different because of which genes are switched on and off. We don’t have a good way to make them go back to being stem cells, with all the possible fates available to them. Plus the stem cells would have to communicate to form a whole new arm. Salamander cells do this, as you saw in the video clip. Cells that were in their bones, muscles, etc, become stem cells again and cooperate to form a new arm.

Scientists have found one gene in mice that lets them regenerate ear tips and toe tips, but activating it only allows regeneration when the mouse is still young. True regeneration will probably take several more genes that have to switch on and off in some complex way. I can’t see the future any better than you, but I predict that limb regeneration for humans is will not be an easy problem to solve. I predict that it will cause unfortunate problems like cancer, which is an uncontrolled growth of cells that stop doing their jobs. One paper predicts that it would take years for a human to grow a new limb.

Why do you think regeneration is possible in amphibians, but not mammals? Can you think of any disadvantages to limb regeneration? Thanks for asking,

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