Claw development and cornification in the passeraceous bird zebrafinch (Taeniatopygia guttata castanotis)

The histogenesis and cornification of claws in zebrafinch embryos has been analyzed. At 10-12 days post-deposition, the epidermis at the tip of the toes forms placode-like anlage associated with a mesenchymal condensation and with a terminal phalange. Claws seem to be modified scales, the dorsal side of which becomes the unguis whereas a ventral scale is the origin of the sub-unguis. At 14-15 days, numerous keratinocytes form the unguis, the corneous layer of which becomes thicker than in the sub-unguis and accumulates beta-keratin and lipids. Keratin bundles are mainly directed toward the tip of the claw and have a prevalent parallel orientation. Unguis corneocytes are thicker and accumulate more beta-keratin than corneocytes of the sub-unguis. Mature corneocytes become partially fused in a compact corneous layer at 17-18 days, near hatching. During growth of the unguis, the embryonic epidermis and beta-keratin cells curve over the tip of the claw and localize in the ventral part of the claw, forming the claw pad. The latter is shed at hatching leaving the pointed claw made of harder corneous layers in the unguis side of the claw.