Abstract
An ultrastructural, immunocytochemical and electrophoretic study on keratinocytes forming the corneous layer of claws in the frog Xenopus laevis has been conducted in order to analyze the process of cornification in amphibians and compare it with cornification in amniote claws. During claw differentiation pre-corneous keratinocytes accumulate dense corneous bundles containing keratin filaments and inter-keratin material. Ribosomes are associated with keratin bundles, suggesting that neo-synthesized matrix molecules are rapidly aggregated onto the keratin filaments, turning the complex into an amorphous, electron-dense, and resistant horny material. Most proteins of the claw are acidic to neutral keratins of 45–66 kDa, but other protein spots outside the molecular weight range of keratins are also present. These proteins of 16–17 kDa with pI at 7.0–7.3 contain little histidine and may represent ‘amphibian keratin associated proteins’ (aKAPs). The association of keratins and aKAPs determine the formation of the hard corneous material of claws and likely of the other corneous structures present in amphibian skin, such as the horny beak and horny nuptial pads. The cornification in amphibian claws, compared to the process of cornification present in amniote claws, shows common aspects, but is more simplified in amphibians.
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