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
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.
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