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 thedifferent cornification. Immunolocalization of acidic alpha-keratin (AK2) of 45–50 kDa in tonofilamentsof the epidermis in Apalone spinifera and absence in the corneous layer where desquamating corneocytesare present supports the biochemical data. Corneocytes shows a weak to absent immunolabeling forbeta-proteins (formerly beta-keratins) of 14–16 kDa while sparse immunolabeled corneous granules areseen in the pre-corneous layer. In the hard-shelled turtle Pseudemys nelsonii differentiating corneocytescontain small level of acidic alpha-keratin while beta-proteins of 10–17 kDa form dense aggregates ofcorneous material among tonofilaments. Corneocytes do not desquamate but remain tightly connecteddetermining 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 differencein 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- andlikely of other proteins in corneocytes of the shell in hard-shelled turtles. In the softer epidermis ofhard-shelled turtles and in the soft-shelled turtle a more rapid and continuous turnover of corneocytesis present and no accumulation of beta-proteins and corneocytes takes place. It is hypothesized that thedermis derived from the carapacial ridge during development remains localized underneath the shellepidermis in hard-shelled turtles and influences the formation of the hard corneous epidermis