The epidermis of snakes produces two types of keratinocytes containing an alpha (soft) and a beta (hard) form of keratin. During epidermal renewal, which allows body growth, six types of epidermal layers are produced before moulting: oberhautchen, beta-, mesos, alpha-, lacunar, and clear. The precise layers in which beta-keratin, alpha-keratin and matrix proteins (loricrin and filaggrin) are produced, have been studied by light and ultrastructural immunocytochemistry. An antibody against chick scale beta-keratin recognizes also snake beta-keratin only in the oberhautchen and beta-layer. This confirms the phylogenetic relationships between avian and reptilian hard (beta) keratins. Beta-keratin rapidly disappears in mesos-cells, where little beta-keratin is present. Antibodies that recognize alpha-keratins (AE1, AE2, AE3) show that acidic keratins (AE1 positive) mainly localize in the most basal layers and basic keratins (AE3 positive) in all layers but disappear in those layers that are keratinized, especially in the beta-layer. The AE2 immunolabelling (a marker of keratinization in mammalian epidermis) is localized in the compact alpha-layer. This pattern is similar to that for loricrin and filaggrin, although in the latter the reaction is less intense and located mainly in the hinge regions among scales. Alpha-keratins and associated proteins disappear in the oberhautchen and beta-cells where beta-keratin is produced instead. This study suggests that the shedding mechanism in snakes evolved after the hard beta-keratin was compartmentalized beneath the soft alpha-keratin to create an intra-epidermal weakness region. The study also suggests that some common mechanism of alpha-keratinization is present in snakes, and the other amniotes in general.
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