system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can a fetus grow a hand with finger nails but if you cut your finger off it won’t grow a new one?
gpt
The presence (and lack) of specific hormones determine what parts of a cell's DNA strand activate (or remain dormant). This basically determines how a cell is going to behave: in how it multiplies and divides, what kind of cell it becomes, etc.

However, once it goes through this process and becomes a specialized cell, it loses the ability to become another kind of cell. It has become specialized and fix. So when you lose a body part, you no longer have cells in that part of the body that have the capability to become the cells you lost.