Salamanders, such as axolotls, hatch in ponds alongside hungry siblings that nibble on them. This may explain why they evolved the ability to regenerate missing limbs and gills. In contrast, humans have a rolling programme of replacing about 10 billion cells per day.
This hints at a possibility that we have inherited the ability to regenerate limbs, yet the relevant bits of genetic code may be switched off or modified. Rapid cell division is associated with tissue regeneration, but it is also a feature of cancer. It is possible that evolution in humans has suppressed rapid cell division in order to combat cancer at the cost of losing our ability to regenerate tissue. Tantalisingly, salamanders regenerate tissue but hardly ever get cancer.
The axolotl is easy to breed in captivity, which has made it the focus of intensive research. When it loses a limb, cells migrate to the site of the wound, turning back their internal clocks on the way. The cells form a blastema, a mass of undifferentiated cells, like embryonic cells or stem cells. Immune cells called M2 macrophages reduce inflammation at the wound site, while connective tissue cells called fibroblasts carry positional information that allows them to differentiate into the appropriate specialised cells specific to their location as the lost limb regrows.
“Salamanders hatch in ponds alongside hungry siblings that nibble on them, which may explain why they regenerate limbs”
Scientists have recently mapped the axolotl genome and this should speed up our genetic understanding of why some creatures can regenerate their limbs.