The development of skin appendages is initiated by the activation of the Wnt signalling pathway in the top layer of the skin (the epidermis), and this signal is refined and maintained by the Eda signalling pathway. Both systems activate two further pathways that are important for cell growth and development: fibroblast growth factor (Fgf) signalling is necessary to ensure that the patterning of the appendages happens correctly and that scales are able to form; the Sonic hedgehog pathway ensures that the epidermis folds properly over the scales. Fish scales are formed from bone, whereas feathers, reptile scales and hair are formed from keratin. However, over time a common ancestor may have stopped using bone and started using keratin to build skin appendages in amniotes.