system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that humans can regenerate skin, bone and muscle after an injury but can't regrow lost limbs or eyes?

It's not clear to me how the body "knows" what the difference between healing a cut in your leg or burn on your hand and the loss of a hand or ear, etc. Obviously the severity of the wound is more severe in the latter case but isn't the process the same (regrowing cells that match the cells around the wound based on the person's DNA)?

As a follow up would it be possible with stem cells to grant the body the ability to regrow any lost body part or would the body part still need to be artificially grown in a lab and then grafted onto the body?
gpt
The regeneration process for each tissue type in our body is very different. Some types of tissues are able to regrow very fast either because the cells they are composed of are proliferative, or because they have adult stem cell pools associated with the tissue. Skin is an example of a tissue that is composed of highly proliferative cells (fibroblasts and epithelial cells). Muscle has an associated stem cell population called satellite cells that are activated during muscle regeneration and differentiate into (that is, change irreversibly into) muscle fibers, while also maintaining their own population. Bone is somewhat special among tissue types in that it is composed primarily of a hard, calcium-based extracellular matrix that is secreted by cells called osteoblasts rather than of the cells themselves, so when bone is injured, osteoblasts secrete more matrix to heal the bone. The adult stem cell precursors of osteoblasts (and several other cell types), bone marrow mesenchymal stem cells, are also found in significant numbers in adults. There is an obvious evolutionary advantage to these tissue types' ability to heal quickly: skin, as our outer layer of defense against the pathogens of the outside world, needs to be able to cover any abrasions quickly, while muscle and bone accumulate damage as they are used (especially muscle - damage and subsequent repair is the mechanism by which muscle grows after exercise). 

Other cell types, such as nerve cells or pancreatic islet cells, are post-mitotic, which means they no longer proliferate under normal conditions. This means that if these cells die, new cells are not formed. Also, the structures you mentioned, limbs and eyes, are very complicated structures composed of more than one tissue type arranged in specific geometrical relationships with one another. Regenerating an entire structure like this requires coordinated action between different cell types. Studying the mechanisms by which these complicated structures are formed during embryonic development, and hopefully discovering ways to apply them to adults using stem cell therapies (either embryonic stem cell therapies or stem cell therapies leveraging the populations of adult stem cells that are present in our bodies), are active areas of research central to a discipline of biomedical research called regenerative medicine.

Source: Bioengineering student at UC Berkeley, with a focus on cell and tissue engineering. Working in a stem cell lab.