When a piece of skin, muscle, bone, nails, or hair gets removed or damaged, those usually grow back like nothing happened. So, why isn’t it possible for hands, or even something smaller like a finger to regrow? Or would a piece of meat just regrow there instead of something useful?
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The body doesn’t know how to build a hand “from scratch” the same way a carpenter knows how to build a house. It only knows how to tell developing tissues to form into many different things, including hands.
Certain genes get activated and certain proteins get made that tell nearby cells what to do.
A tissue on the side of the embryo called “lateral plate mesoderm” starts growing into little buds and forms a substance inside called mesenchyme. This differentiates into cells that will become cartilage and bones, and at a certain stage, parts of it get told to die in order to make the spaces between fingers.
Then these “chunks” of tissue near the midline called somites (paraxial mesoderm) have to form other chunks called hypaxial myotomes, which start this sort of “parade” as they migrate out into the limb and are followed by growing spinal nerves.
During the whole process, lots of developing tissues are all communicating with each other and mutually telling each other what to do. These tissues don’t really exist in the same way in an adult human. Having lots of rapidly dividing and differentiating tissues isn’t the way the human body works.
TLDR: The development of many structures and organs is dependent on contextual signals and communication between developing tissues that only exists in the first few months after fertilization.
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