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
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“Embryogenesis” is the study of how a single cell becomes a fully formed human. It’s an incredibly complex process involving many different genes and specialized cells and we’re just scratching the surface of fully understanding it.
Some of the cells that an early embryo has that have the potential to become anything are called “pluripotent stem cells” and they only exist in embryos.
Regrowing organs and tissues that are a genetic match for patients who need them is the holy grail of this area of medicine and science but we’re not quite there yet.
Before we are born, the cells in our bodies know how to do lots of jobs. As we grow in our mama’s belly our cells get assigned to do specific jobs like “be an eyeball” or “taste food”. After we are born, the cells in our bodies only know how to do the very specific jobs they were assigned. As a kid or an adult, if we lose part of our body, there aren’t any cells left that know how to do that job.
Generating these parts requires specific, undifferentiated cells, in a very carefully orchestrated and intricate process of development. It’s largely designed to all work cooperatively and integrate numerous feedback loops of its neighbors and the larger organism as it shapes itself.
Afterwards, you are left with cells with much higher differentiation and commitment to what it is. At that point, your body is more about patching and surviving than true regeneration.
Regrowing limbs and organs in an established and developed organism is a very, very different context, and one that lacks the signaling cues and needed cells in the right place to carry it out.
It’s just not in our cells’ instructions. Someone losing their heart and surviving long enough to potentially grow a new one doesn’t happen often enough for the mutation to “stick” and play a role in our evolution.
The salamander being able to regrow certain parts is an example of this as well – A lizard narrowly escaping after getting caught by one of its limbs is more likely to survive and reproduce.
We can, at a cost. It’s called Regenerative Medicine.
A long time ago in our evolution we probably could regenerate entire limbs or organs, but the ones that quickly closed wounds with clotting and scarring outlived the ones that left wounds open to regenerate (probably died of infection).
Doctors are using what is called extracellular matrix (**ECM**, a scaffold made of cells within the body, or artificially extracted from pig bladders) to create a regrowth medium to entice stem cells (your body’s “I can be anything” cells) to regrow tissue. The matrix promotes vessel growth, and the stem cells fill in the gaps with tissue.
Kidney damage? Remove the damaged tissue, Put ECM at the site, and your body uses your own DNA map and surrounding tissue to regenerate your kidney to specifications, following it’s own blueprint.
The costs:
1. Wound must be left open, so vastly increases the danger of bacterial infection.
2. It takes a very long time to regenerate some things. 6-9 months for an organ. A human can live hours without a liver, for instance. Even a partially donated liver is a better option, because they grow back.
3. We are still figuring it out, so the technology is in it’s infancy. Vets do use it on animals, and it’s quite successful.
Have a read. Warning, **not ELI5**: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9973391/#:~:text=Discarded%20human%20donor%20organs%20can,for%20organ%20engineering%20using%20hPSCs.
Vets: https://www.sciencedirect.com/science/article/abs/pii/S1534751604000423
Human successes: https://www.popsci.com/scitech/article/2008-05/man-regenerates-finger/
It’s actually a huge mystery. Mammals in general are not good regenerators, unlike a lot of other kinds of life. One idea is that our immune systems disfavor regeneration because the process involves creating cells that are highly unspecified (cells that have the potential to become lots of other kinds of cells i.e. pre-differentiated). Another process that involves cells like that is tumorigenesis. So it could be a tradeoff; complex mammalian immune systems that survey and prevent cancer could also be what stops us from regenerating. But let me emphasize, this is an idea; the reason is not at all well understood.
Short answer: regrowing limbs doesn’t work on animals that live longer than stuff like lizards. The reason is that it uses similar mechanisms to cancer growth.
That’s an apples to oranges comparison.
Humans are designed to create more humans, all organic species are. Humans are not designed to grow back organs with few exceptions (liver).
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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