Body growth in spiders (indeed in all arthropods) happen by jettisoning the tough outer shell while a new skin grows beneath, says Richard Jones.
This is completely on growth from a new batch of embryonic stem cells. The regrown leg may lock slimmer or smaller, and it may take two or three moults to regrow a fully functional new leg.
Why can't humans regrow limbs?
In humans, stem cells become pre-determined during body growth at the embryo stage, and are only found in adults in places like the bone marrow (to generate new blood cells) and the gonads (to create sperm and eggs). They also occur in the skin's epithelium layer, and although humans cannot grow a new limb, scar formation is still pretty amazing and. in some cases. life saving.
