A comparative analysis of the cat claw, horse hoof, and human fingernail

Abstract

Earlier authors (e.g., Boas, 1931) have tried to homologize the skin modifications in mammalian cornified digital end organs (claws, hooves, and nails), but have considered only major parts, such as the wall and sole. A recent analysis of the cat claw (Homberger et al., in press) and horse hoof (Bragulla & Homberger, 2007) revealed that the subcutis, dermis, and epidermis are regionally modified to meet functional demands. The structure of digital end organs is, hence, more complex than anticipated. For example, the subcutis can form a hydrostatic cushion or ossify to strengthen the attachment of the dermis to the bony distal phalanx. The dermis forms a papillary body, which increases the dermo‐epidermal interface, enhances the supply of the avascular epidermis via diffusion, and strengthens the dermo‐epidermal bond. In addition, the papillae, ridges, or lamellae of the dermal papillary body often influence the architecture of the modified epidermis that forms the cornified digital end organs. Furthermore, several hard‐ and soft‐cornified segments can be identified and distinguished within the cornified digital end organs. The local and integrated mechanical properties of the digital end organs result, therefore, from different combinations of comparable building blocks of modified skin. Hence, the traditional homologizations between claws, hooves, and nails need to be re‐evaluated at a more detailed level.