Ultrastructural immunolocalization of alpha-keratins and associated beta-proteins (beta-keratins) suggests a new interpretation on the process of hard and soft cornification in turtle epidermis.

The epidermis of soft-shelled and hard-shelled turtles has been compared to determine the origin of the different cornification. Immunolocalization of acidic alpha-keratin (AK2) of 45-50 kDa in tonofilaments of the epidermis in Apalone spinifera and absence in the corneous layer where desquamating corneocytes are present supports the biochemical data. Corneocytes shows a weak to absent immunolabeling for beta-proteins (formerly beta-keratins) of 14-16 kDa while sparse immunolabeled corneous granules are seen in the pre-corneous layer. In the hard-shelled turtle Pseudemys nelsonii differentiating corneocytes contain small level of acidic alpha-keratin while beta-proteins of 10-17 kDa form dense aggregates of corneous material among tonofilaments. Corneocytes do not desquamate but remain tightly connected determining an increase in thickness of the corneous layer that becomes mechanically stiff and resistant. Since both species possess beta-proteins in shelled and non-shelled areas of the epidermis the difference in hardness of the corneous layer is not due to the alternation between beta-keratin versus alpha-keratin. Mechanical resilience of the corneous layer derives from the accumulation of alpha-keratins, beta- and likely of other proteins in corneocytes of the shell in hard-shelled turtles. In the softer epidermis of hard-shelled turtles and in the soft-shelled turtle a more rapid and continuous turnover of corneocytes is present and no accumulation of beta-proteins and corneocytes takes place. It is hypothesized that the dermis derived from the carapacial ridge during development remains localized underneath the shell epidermis in hard-shelled turtles and influences the formation of the hard corneous epidermis.