system
Given the following post, respond as if you're commenting on Reddit:
human
why don't humans regenerate limbs?

We all grew limbs when we were in the womb originally and from what I've heard after some amount of time there isn't a single cell that is still with us from before.  Why can't we regenerate limbs?  It seems that the ability is there to do so. 

and will science ever be able to progress to where we can regenerate limbs?
gpt
In short, because the cells mature to a point where they lack the capacity to regrow. Remember, there are four tissue types involved in the process.

Epithelial cells by far would have the highest regenerative capacity, but remember, in a burn, if you lose the basal level, skin won't regrow. That's where the stem cells are, so people who have lost that level require grafting. Without skin, the arm would be a huge liability to the organism, as it would provide an easy route for infection.

Next, there's connective tissue. Ignoring all of the other types present, we'll focus on bone. The long bones of the arms and legs develop via endochondral formation, where the "growth plates" are made of a cartilage matrix that expands and slowly calcifies. The chondrocytes die and are replaced by osteoblasts as we mature, leaving us with bones. Bones can be repaired, but not regrown. 

Muscles are also going to be a pain. Skeletal muscle does have some regenerative capacity, it is limited. Hypertrophy of the cells is the normal response to working out, meaning that the existing cells grow. Hyperplasia, the act of cellular replication, is usually not a good thing in the case of muscle, but would be required for regrowth. 

Nervous tissue, which would allow the arm/leg to have any function, is the least regenerative. There is some evidence of nervous stem cells coming to light, but they clearly have a very limited capacity to work, considering the permanence of brain damage and paralysis. Even if you could grow the bone, skin, and muscle, without the nerves, what's the point?

As each of these tissue types matures during embryogenesis, they lose the capacity to revert to a less mature state. What we refer to as "adult stem cells" are cells with the ability to replicate, but only within the cell line that they have chosen. They can't proceed backwards to pleuripotent embryonic stem cells under normal conditions. There has been experimentation with reverting cells ex vivo that has shown some success, but this is mostly being researched for things such as regenerating heart muscle or nerves (things without regenerative capacity). So is it possible that we may progress to where we can regenerate limbs in medical practice? Maybe. But it's nowhere near a reality, seeing as we are still struggling with regenerating one tissue, let alone synchronizing the development of all of the tissues in a limb. 

And all of this is not to mention the risks with reverting our cells to stem cells. Keep in mind that a reversal to a less mature state is one of the hallmarks of cancer, meaning that minor errors on a genetic level could predispose the patient to cancer, and the more we have to do, the more likely we will be to make that little mistake. (Our cells make mistakes in replication on their own about 1/1,000,000 times.) While it may seem a worthwhile endeavor at the time to give a person their arm back, giving them malignant cancer that kills them before the limb can even fully grow would make the whole thing moot.