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
I remember someone explaining this in a story once basically your body is a house with four walls and some windows if you break a wall it knows to fix the wall but it doesnt know about the window.
In true ELI5 fashion;
when we start to develop our body has a bunch of tiny little robots. These robots went to school for a very specific subject, and then given specific jobs regarding that subject to grow into whatever career they are given. They are good at their job and even have ‘kids’ that run the family business to fix minor issues, but in the end after we are born, those robots cannot change jobs and only know how to do that one thing, and we are not left with a high number of ‘unemployed’ robots to give the ol ‘you need to get a job’ speech. But, those ‘unemployed’ robots still do important task such as renewing things inside our body. The robots with specific jobs can do their jobs well, but if any major problems arise, they again, can only do their job so well in some cases.
Our liver and skin are basically the only organs we can regenerate quite well, and it makes sense too since the liver and skin can get taxed very easily, all your blood essentially filters and gets cleaned by your liver, and it would be pretty bad survival wise if you just never were able to regrow patches of skin. Other organs have a lot of trouble regenerating things, they can heal minor wounds, but even then they can struggle with that.
I heard a good analogy once:
Imagine a normal housefront that has a window and a door. Now smash in the window and break the door. Easy to fix, even though you’ve never seen the house before, yes? Just replace the window and fix the door.
Now get another house, smash in the whole wall so there’s nothing left but a gaping hole. Now order in a repair company and tell them to fix it as it were, but you don’t have any reference or pictures to show how it used to look.
All they can do is build a wall without any windows or doors to make the house liveable again, because they don’t know what it used to look like, where the door was or how many windows etc.
1. The body is not designed to survive *everything.* It is designed to survive the most common problems that are easiest to fix. It would be nice for animals (thinking of mammals) to be able to regrow a limb that was torn off. But 99% of the time, they would die from the initial wound, or not survive the regrowth process (they would starve or be eaten first). So evolution could not select for a regrowing feature.
2. People describe DNA as a “blueprint” for the body, but that isn’t true. DNA is closer to the literal, detailed, step-by-step instructions on how to build the body from scratch. So if you lose a body part, your DNA does not “know” what to do.
3. Many things are easier to replace than repair. Think about a fan. If a blade in the fan breaks, we could replace the fan blades or even build a custom blade piece and solder it on. But it is much easier, if more wasteful, to buy a whole new fan and junk the old one. In a way, this is our genes’ approach to our bodies. If something big breaks, give up on that body and let someone else take its place.
The purpose of our bodies is to reproduce, as in “keep this group of genes still in living beings”, or “build new entire humans beings from scratch”.
So it begins structuring a body with cells that are given specific tasks to keep it going until it can reproduce. Since evolution is what drives the ways the organisms are built to survive until they reproduce, throughout history the “regrow limbs” skill did not significantly impact humans, at least not in the same way it did some lizards and starfish.
The reason for why it cannot grow back whole limbs or some other organs is because it does not need to. It has the capacity for it written on its DNA, but to put that into work it would be unnecessarily taxing to the body, specially one that can reproduce without that ability being frequently needed.
Part of the mystery of how we are formed is the turning on/off of this in our cells. People with deformities and parts that don’t grow right can be attributed in part to things not working correctly if it’s not attributed to a genetic abnormality but a physical barrier. But in essence it’s like telling cells to reproduce and form the structures by following different blueprints on how to regrow whatever it is you want.
I think the hardest part of studying this is the manner one must go about doing this… by examining growing humans. So the limitations are all going to be around the ethics of this because it’s essentially a growing human you’re now having to study in a way that could end its life and thus in steps the morals of things.
But of course there are countless species we can study this on mammals wise as the process is similar and feel less inclined to the morals of it not being people. However the difficulty is that it’s different to study a subject from when it’s just one cell vs when you’re an adult and all of that processes has been dormant since you were originally in the womb.
So instead we try and figure how creatures like starfish can grow back an arm to the same shape and function to what it lost, a rather amazing process that is absent in most higher level creatures. With the exception of certain lizards who can regrow a lost tail.
It is rather amazing and maybe if we can figure out the secret to regrowing entire organs, one could speculate that a replacement organ could be grown is a possibility. But given how long it takes for a baby to get to the size it does – one would realize that it’s not a speed process and if the need was dire, you probably couldn’t wait.
Your body *doesn’t* form an entire human body from scratch, it just serves as a good host for an embryo to develop.
Think of the genome like a library. If you’re a newly fertilised embryo, the librarians heard you were coming and had a massive reshuffle to make sure you can check out any book you need to. The library is only fully open for a small number of cell divisions right at the beginning of fertilisations at the beginning of the formation of the embryo. After that, the librarians get to work closing parts of the library off.
The librarians think it’s a bit dangerous for most other people to be able to check out any book they want to since they could be reading books they really shouldn’t be, so they go around and close bits of the library off. So now when someone like a multipotent haematopoietic stem cell comes to the library, it can only check out books related to making cells of the blood and immune system.
But the librarians need to be even more careful, so they close off even more of the library. Now when a mature CD8+ T-cell comes to the library, it can only access the books about what a CD8+ T cell should do. Or when an astrocyte comes to the library, it can only access books about what astrocytes can do.
Each cell in your body is an individual library with different parts of the genome accesible. Every cell division from an embryo to something like a muscle cell closes more and more of the library off.
All the cells in your arm can only access books related to what they should be doing on a regular basis. You know – arm stuff. Nowhere in any of the books they can access does it talk about how to regrow an arm. So they aren’t able to do anything about an amputation.
It seems there’s something in the embryo that tells the cells how to form and when the job is done( when you’ve grown), it retires. We need to figure out what that thing is and replicate it.
Because you don’t want a random liver growing out of your elbow.
Growth and differentiation happen under carefully programmed conditions and cells actually “talk” to each other through various signaling to find their position/identity. They don’t have a top down map of the body, so they find their position relative to the signals put out by neighboring tissues, and most of that natural programming is calibrated for the first few weeks of fetal development, or other specific stages of growth.
When the systems controlling/restraining that programming shut down you get cancers.
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