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
In school we learn about the basic structure of a cell: it has an outer membrane containing some goo (called “cytoplasm”) and some consistent structures inside, like the nucleus that contains the DNA. This basic model of a cell glosses over the complexity that there are very different kinds of cells in the human body that do very different jobs. Neurons are different in structure and function from muscle cells or bone marrow cells.
But all these different cells start off from a common root source: the fertilized egg. So how does that happen? Well, when the egg is fertilized, it starts dividing, producing a cluster of cells called an embryo. These earliest cells are not the specialized cells that develop later, but rather a kind of undifferentiated cell we call an “embryonic stem cell.” To start with, we just need a lot more of those. Specialization follows a bit later.
When the time comes, the DNA of those stem cells contains instructions for how to turn themselves into specialized cells when they are exposed to particular chemicals in particular concentrations at particular times. The DNA also contains a whole “body-patterning” process that controls where and when the release of these chemicals starts and stops. The developing embryo knows up from down, back from front and right from left by establishing gradients of these chemicals, stronger in one direction and weaker in the other.
As development progresses, a lot of the specifics are delegated. The earliest developmental differentiation turns the embryo into sort of a digestive tube with a mouth end and a butt end. This in turn activates developmental systems that eventually sprout a head near the mouth end and legs near the butt end. You’ve got to delegate this way because after a point there are just too many cells to coordinate it all with one centralized process.
An important part of this body-patterning process is that it needs to stop at the appropriate time. A lot of birth defects result from errors in body patterning. At some point your developing body needs to be able to say “okay, that’s enough fingers now” or “this blob of cells is now officially a liver.” Due to the delegation of developmental specialization, the overall body-patterning system no longer controls what happens. Your liver cells are liver cells forever, and they take over responsibility for their own growth and healing locally.
If your liver is removed after that point, there is no functioning mechanism remaining that tells the neighboring organs (or your body as a whole) that there ought to be a liver in that hole, or how to grow one. Among our ancestors, losing your liver was just going to be immediately fatal. Retaining the option to grow a whole new liver probably would have led to errors that resulted in you sprouting new livers where they didn’t belong far more often than the likelihood you’d survive losing your liver long enough to grow a new one.
There *are* examples of animals that can regenerate lost body parts, like lizards that can detach and re-grow their tails as a defense mechanism, but that’s something that evolved particularly in those lizards. It’s not a general ability to re-grow any missing body system, but rather that the tail stump part of the lizard retains the specific ability to grow a new tail. On the other hand, multicellular animals that *can* perform universal regeneration, like planarians, are extremely simple organisms, and there’s no reason to expect this could scale up to something like a mammal.
Perhaps there is some way we could re-activate these developmental systems. Researchers continue to investigate ways that specialized cells might revert to stem cells, or to identify other forms of adult stem cells. In an ideal circumstance, transplant of donor organs would be rendered obsolete if we could figure out a way to get enough stem cells and instruct them to develop into a new liver that is, genetically, 100% *your* liver. In principle all the right genes are still tantalizingly lurking there in every cell. We just don’t know what to do to induce them to do what we’d want. It’s a very complicated process that, as far as developmental biology is concerned, was only supposed to happen once and in one direction.
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