Eli5: if our bodies are able to form entire human beings from scratch, why can’t it grow back organs that have been amputated?
In: Biology
Cells grow and reproduce by fairly strict procedural rules, and the process of growth is an important part of how they define what should grow next.
Lop off someone’s arm, and the cells at the stump are already in “I am fully developed” mode, and will only be reproducing for the purpose of replacing themselves as they age out, population-maintenance rather than growth. They don’t have any means of detecting that they need to shift to grow at a higher tempo for regenerating.
You can imagine that as a baby, you have a series of early cell-division behaviours that result in a human-shaped blob, which then grows out bones and muscles and tendons in a loose fashion, which tightens up into the final configuration, and then from there on, those cells are self-reproducing and not aware of their neighbours, they only extend in density and quality.
Exception goes for things like Skin, where it’s advantageous to grow skin over injuries, and to some extent muscles and such can regrow in similar ways, but it’s important to mention that this kind of mass-production of the body typically takes place when there isn’t actually very much body to build. A foetus is really really tiny, and even a newborn baby is only around 6lbs of material and took nine months of dedicated effort to build with the benefit of a liquid support environment and unlimited supplies of nutrients.
For more reading, you might want to look into [L-Systems](https://en.wikipedia.org/wiki/L-system) for example, which are a series of basic rules which any given cell follows, and can produce complex and intricate structures with semi-predictable patterns.
Basically the underpinnings of how DNA can be the same for any given cell, and produce such wildly different structures as fingernails, kidneys, teeth, eyes, skin and brain-matter.
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