A comparison of the epidermis and pigment cells of the crocodile with those in two lizard species

Keratinization and pigmentation in Crocodilus niloticus skin were compared with the conditions in the lizards Lacerta viridis and Anolis carolinensis. The epidermis, both in the crocodile and in lizards, is arranged to form a surface pattern of scales and narrower intervening hinge regions. Similar keratin-bound substances were found in the crocodile and lizard stratum corneum. Nevertheless, the greater uniformity in histological structure and in distribution of chemical substances throughout the depth of the crocodile stratum corneum was in marked contrast to the lizards, which showed morphological differences, and differences in intensities of chemical reactions in the horny cells laid down early and late in each keratinization cycle. In the crocodile, keratin-bound S-S and SH are uniformly distributed in the horny scales, but in the lizards the superficial cells have most S-S and the lowermost keratinized cells most SH. The loosely arranged horny cells in the crocodile are shed in small flakes as in mammals, in contrast to lizards which undergo periodic sloughs of a compact stratum corneum. In the lizards, the intermediate layer between two horny layer generations contains no detectable S-S and is probably unkeratinized, so that when these cells die a fission zone is formed. The crocodile scales each contain a raised pigmented papule in which melanin is introduced into the epidermal cells, and keratinization is also different from the neighbouring area. Guanophores and lipophores are absent in the crocodile, although present in the lizards. All contain prominent dermal melanophores.