system
Given the following post, respond as if you're commenting on Reddit:
human
Why do certain parts of our body reproduce 
themselves when destroyed (eg skin) and others 
don't?

For example, if i rip or burn the skin on my fingertips, it will grow back. Or scars are somehow covered up.. But why if we cut a person's finger, or a whole hand it will not grow again..? Can this become under certain circumstances..? Any other examples more microscopically ?
gpt
Some parts of our body contain cellular machinery to heal wounds, and some do not.

In the skin, if you have a deep cut, the stem cells (basal keratinocytes) in the stratum basale need to divide and reform into a single layer to start regenerating skin cells in the epidermal layer above them.  If they are too far separated, this regeneration will be incomplete, which is why stitches are sometimes necessary.

If you cut off a finger, there's no basal structures that can regenerate and the architecture is so complex, simple cell growth will not suffice.

Other areas in your body - like your GI tract - work in much the same way.

Your liver can take quite a bit of damage and still regenerate - from as little as 25% of the original structure - largely because other liver cells will proliferate and fill the space, restoring function if not the original architecture.