My mind keeps going to the Charmin’ Toilet Paper ad and the slogan ”Try a tissue that leaves fewer bits behind.”

I don’t think that’s what you’re going for here, though.

the appendix

tonsils.....? My guess. Altho why you'd want them to grow back....probably not the answer...

Your spleen

Liver? I've heard of people donating part of their liver.

Rusty

Hair on the top of my head, not like my ears.

Here's another question: What is missing from this sentence?

"This will grow back there was tissue left behind during the removal process."

I think the word "if" is missing, but I'm not sure.

If so, my guess is wisdom teeth.

A reptile's tail, works for chameleons

I heard that about tonsils.

Well my Adenoids grew back after 40 years, so I will say that. Had them removed again at 58 years old.

I remember now that it was my mother's tonsils that grew back instead of her wisdom teeth!