why our skin and liver, the largest organs in our body, regenerate but other organs can’t?
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Imagine your body as a wall of a building. Then something or someone makes a big hole in it. When body tries to repair it, it looks around the hole and tries to make something similar from the same bricks. Liver is big for sure, and skin is also, but they are not very “detailed”. It takes time to repair them, for sure, but it is possible. Imagine that your eye is a window in the wall. Or that’s a part of the wall with the graffiti on it. If it is destroyed, body will look around and repair the hole .. But it has no idea this part of the wall had a window. It will repair it with the same bricks. That’s why eye can’t regenerate. Or why we can’t grow finger again: imagine that half of the wall was destroyed. Body doesn’t know how large the wall was. All it sees is some broken in half bricks. It will restore them, but won’t go further.
Also it is much harder to live without the liver or heart than without finger or eye. Eye damage is rare. Statistically the mutation that helps liver/skin regenerate will be much more helpful for reproducing than mutation that helps an eye regenerate. Also – even a small mutation that helps regenerate liver will help you. For an eye regeneration mutation to have any evolutionary benefits it must be a huge (HUGE) mutation that almost can’t happen in such complex creatures as humans are.
Most organs regenerate to some degree, some more than others. Still the regeneration isn’t perfect. Scarring usually occurs or the regenerated tissue is not exactly like the damaged one. Sure livers regenerate and they become as big as they were but that doesn’t mean their functional capacity is the same. Bones heal too, and muscles and ligaments but often injury to those means chronic complications and their full capacity may never be regained. It’s the same with skin. Sure a paper cut heals pretty well but serious injury to skin leaves scars and often nerve damage and partial or complete loss of sensation.
Evolution only figures out how to do things that we need in order to have kids and raise them until they can have kids. We needed those organs to regenerate in order to do that, but other organs didn’t need to regenerate so they didn’t get evolved to do that.
The spleen can! My brother had half of his removed after a car accident when he was little and it grew back at least partially.
It’s easier to cover a damaged white wall in more white paint than it is to repair the Mona Lisa. Organs which are fairly uniform in construction are basically just painting over the cracks.
Esophageal lining, stomach lining: AM IA JOKE TO YOU?
You’d surprised how often this question comes up at the “meetings”. Anyways, It’s best not to sugarcoat it. Those “organs” are actually parasites in a symbiotic relationship with human beings. Things is, if the doctors can’t tell the difference, what chance does Joe Schmoe have?
Well then, how do they get into us without the host knowing? The parasite’s eggs are microscopic and go along for the ride, so to speak, when you go for a ride, if you get me.
They form as the embryo forms. They filter out the bad stuff, eating it, and keep us alive in the process. They can die, of course, if we overtax its ability to filter out things or, have an accident resulting in traumatic injury to that “organ”.
They’ve been with us for as long as apes and humans have existed.
There are two ways an organ can regenerate. One is through a reserve of stem cells. Stem cells have the ability to create new cells; sometimes they can create multiple different types of cells. They are called “immature” or “undifferentiated” bc they do not resemble their more mature forms. People are most familiar with stem cells in the bone marrow, which create different types of blood cells. But there are forms of stem-like cells (progenitor cells) in other organs, such as in the skin, where the bottom layer of the epidermis (basal cells) gives rise to new cells.
The second way to regenerate is by having mature or “differentiated” cells switch into replication mode through complicated cell signaling. This is much less common but is notably seen in the liver.
The reason some organs cannot regenerate is because they do not have a stem cell/progenitor cell reserve and they do not have the ability to switch back into replication mode. These cells are called “terminally differentiated” – the most used example are muscle cells in the heart.
Tissue that gets regenerated regularly (like skin and liver, though skin is MUCH faster) tend to also be tissues that get damaged regularly or are regularly exposed to damaging materials. Organs that don’t do this (brain, heart, etc.) also tend to not get exposed to damaging materials regularly and therefore do not need to regenerate as much. Regeneration takes a large amount of energy and runs risks (increasing chance of cancer is probably the most obvious), so the body is going to evolve to reduce it where possible and increase it where necessary to balance out energy spent vs. risk. It goes deeper than this, i.e. physical constraints, but this may extend beyond an ELI5
Source: Am neuroscience PhD student
Our skin is there to keep the pathogens outside of our body (and there’s a lot of them!) on the outside so it can’t kill us. Our liver is there to keep the potentially toxic food (and there’s a lot of it!) that we put inside of us from killing us. Both can take damage by the nature of how they perform. That being said, our body does heal itself and not just those two organs. We cannot regenerate limbs like some species, but as long as the damage isn’t too traumatic the body is capable of healing itself to an extent.
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