Understanding the patterns of normal skin and its appendages is a persistent concern in cutaneous biology. In mammalians, many patterns are attributed to reaction-diffusion mechanisms that determine interactions between the epidermis and dermis during embryogenesis.
By contrast, the scales of the Nile Crocodile’s head result from a different physical interaction between dermis and epidermis. The January 4, 2013 cover of Science features the eye and surrounding skin of a crocodile. Many polygonal scales with sharp borders reminiscent of stratum corneum scales are shown in the article’s supplementary material. The cracks are often incomplete, exhibit a relatively random pattern, and are nonoverlapping — and thus different from stratum corneum scales.
The cracks form and their patterns are fixed during embryonic life; they are confined to the head region, which grows more rapidly than the crocodile’s body. Its beta-keratin epidermis resists pressure from the growing dermis — until cracks form successively and then propagate. The modal number of polygon scale edges reported was five, but the number of edges was distributed significantly.
The reddish-brown dots that often appear near the edges of scales are dome pressure receptors (DPR); connected to cutaneous nerves, they are not crossed by the epidermal cracks, and crocodiles use them to hunt in dark and murky waters. DPMs seem to constrain the cracking, and edges travel where DPR are least dense.
All scales are not created equal: on the crocodile, at least, body scales are similar to what is seen in the skin of mammals. Those interested in skin patterns should read the article and determine whether if physical factors affect the genesis of the shapes they study.
Image used with permission (Trevor kelly/shutterstock.com)