If we make skin and muscle cells when we heal cuts and heal/generate bones after breaking them, why wouldn’t we be able to grow a finger if one is cut off?
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I no longer recall the specific genes involved (it’s been a while since my days studying evo-devo at uni) but generally, there are certain genes (such as PAX6, HOX genes, among others) which are highly upregulated during development (when you are in your mother’s womb). They direct where cells form tissues, to what shape they form in etc. which essentially becomes your human body plan / body parts once your embryogenesis concludes.
Once that embryogenesis stage is over and you are delivered, these genes generally get downregulated – and in any insult occurring thereafter, they can no longer “help” you reform the lost body part.
This is generally the “rule” amongst mammalians and every other species with the exception of the axolotl (Ambystome mexicanum).
Though I’m no longer updated in the current studies on that species, but as far as I know it is being studied as a model organism of organ regeneration, possibly to learn how to apply it to humans.
**Anyone currently in the field of developmental biology may correct any errors I might have mentioned. Feel free to add any updated insights as well.
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