Examples of regeneration abound in the animal kingdom: lizards and salamanders regrow limbs and tails all the time, ditto for starfish. https://blogs.scientificamerican.com/guest-blog/regeneration-the-axolotl-story/
Planaria are even notorious (and perhaps unwilling) participants for experiments in growing two heads (https://www.youtube.com/watch?v=roZeOBZAa2Q). Not that we want to have two heads, but why can’t humans regrow lost organs, arms or legs?
While some of the cells in our bodies do have regenerative capacities—skin heals, the lining of our gut, our liver—they do so in a limited fashion. The classic credo in biology is that the more specialized the function of the cell or tissue, the less its capacity to regrow. As humans are way up in the evolutionary ladder, a lot of our cells have crossed the differentiation point of no return: you may grow some of your hair back, but a severed finger remains a stump.
Our increasing knowledge on stem cells—and their potential to differentiate-- have made more complex tissue regeneration a possibility. In fact, Dr. Levin in his work has proven that bioelectric signals trigger cell and tissue differentiation. Read about his success in electrically stimulating regeneration in amphibians: https://www.popsci.com/body-electrician-whos-rewiring-bodies
An arm or a leg is an intricate combination of skin, bone, muscle, nerve and vascular tissue that all have different functions. The trick is finding the correct signals to stimulate the right progenitor cell in growing into these specific structures.
Once these signals are unlocked, the remaining obstacle is how to keep this process going—and that involves counteracting our own innate healing processes. When the body senses injury it attempts to seal off any exposed areas by dumping collagen into the area that eventually becomes scar tissue. This may be effective in closing off the wound, but it consigns the injured area to a non-functional fate.
A solution is to keep the ‘healing’ area within a hermetic environment where it is conducive for tissue growth. Keeping the growing limb in this portable ‘nutrient bath’ promotes the healing process while keeping it protected from infection or injury.
This theoretical model has been proposed: https://www.popsci.com/how-to-grow-an-arm
Current options for patients who have lost limbs to either disease or trauma remain unsatisfactory. Surgical repair or re-attachment procedures are still fraught with complications; prostheses still cannot adequately restore either form or function. Within the next 25 years, researchers foresee patients being able to regrow their own limbs within a year’s time from amputation.