Development, differentiation and toughness in lizard claws derive from two types of cysteine–glycine‐rich proteins

The present study reports the formation of claws in lizards and their cornification. From a flat epidermis with lose mesenchyme, the skin forms undulations that become symmetric domes and later asymmetric for the elongation of their proximal part that gives rise to the outer scale surface. At the tip of digits claws develop in association with a mesenchyme that surrounds the cartilage of the last phalange. Beta‐cells of claws appear 2–3 embryonic stages before those of scales formed in other body regions, including digital scales. The epidermis of the developing claw initially forms 3–4 embryonic layers under which elongated beta‐cells accumulate long bundles of corneous material, immunopositive for CBPs. Claw beta‐keratinocytes incorporate tritiated proline and accumulate numerous, oriented corneous bundles that rapidly occupy the entire cell. The bundles are electron‐dense under transmission electron microscopy, suggesting that, in association to CBPs, other proteins are synthesized and increase the mechanical resistance of claws. Immunolabelling for cysteine‐rich IFKs, HA and HB, shows that these keratins co‐localize with CBPs only in claws but are low to absent in digital scales and adhesive setae. This indicates that the hardness of claws mainly depends from the association between these different proteins rich in glycine and cysteine.