Why our cells regenerate the hard way

We lose billions of cells from our body everyday. It's natural to lose them, and luckily, we can naturally replace them. But when our cells regenerate, but they don't do it the easy way. The easy way would be for similar cells in the location to divide and replicate themselves to fill in for their lost neighbors. That would be a very fast and energy-efficient way of regenerating. But that's not the way it works, as summarized in a new report in Nature.

When dead cells are replaced, they are replaced from scratch. Each kind of cell in the body has its own dedicatied set of stem cells. To keep up with the need for replacement cells, stem cells act as the seeds from which new cells for that part of the body are sprung. By dividing, the stem cells initiate a series of cell generations, each new generation more speciallized than the one before it, until the just the right replacement cell is produced. This process takes a lot more energy and time than a single cell division.

Building replacement cells from scratch is safer for the body; it guarentees that replacement cells will not have the mutations of the cells from which they divided. Why is this safer? If the dividing cell was deceased, or if it were a cancer cell, then deceased cells would divide and quickly multiply beyond the organism's control.

Futhermore, if our cells were to regenerate by a single cell division, single organisms would evolve as they aged. For example, consider a frog genetically built for a cool, wet environment that moves to a hot, dry environment. As the cells of this frog are naturally replaced over time, the cells that are better suited for dry heat will out-complete the others. And with each generation of new cells, the whole frog would evolve towards the hot, dry environment. Within its single lifespan, this organism would do what is not possible: evolve into a very different one. A species cannot evolve in one generation; it must evolve over a long series of generations.

So, for an organism to make a replacement cell, it must be built from scratch from stem cells; it cannot make cheap copies of ready-made cells.

Interestingly, there is an exception to this rule, found in cells of the immune system. The cells of an organism's immune system must evolve within the organism's lifespan. Take our frog for example: when the frog's immune system encounters a decease it has never handled before, immune cells capable of fighting the decease can directly divide themselves to increase their numbers. It does the frog no good if the frog's descendants' evolve to fight the decease. The frog's immune cells must evolve during the frog's lifetime, or the frog won't stand a chance against the new decease. Does this mean that cells of the immune system are more vulnerable to decease, given that deceased immune cells are can divide and spread themselves? Yes. This is why deceases that attack the immune system are so deadly. link