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Keywords:

  • alligator;
  • alpha-keratin;
  • beta-keratin;
  • development;
  • epidermis;
  • immunocytochemistry;
  • ultrastructure

Abstract

Using specific anti-β keratin and general anti-α keratin antibodies, keratins were located in the epidermis of the alligator during the final developmental stages by ultrastructural and immunocytochemical methods. The maturation of the bilayered periderm (= embryonic epidermis) coincides with the disappearance of cell organelles, including the 25–35-nm-thick coarse filaments, and the coalescing of α-keratin filaments into a compact mass. The plasmalemma of peridermal cells forms a 15–25-nm-thick electron-dense corneous envelope. These changes start at stage 25, about 3 weeks before hatching, and continue until hatching when the embryonic epidermis is shed. Immature β-keratogenic cells beneath the embryonic epidermis accumulate immunolabelled β-filaments which are packed into thin, electron-pale β-keratogenic cells in the corneous layer. Together, electron-pale and electron-dense materials form a compact 3–4-nm filament pattern of β-keratin. Melanosomes from epidermal melanocytes, incorporated into β-cells, give rise to the banded skin pattern of hatchlings. β-keratin production is much reduced in the hinge regions, where many α-filaments remain packed together with lipid droplets or mucous granules into thinner, more electron-dense, α-cells. The keratinaceous material of the α-cells is mostly concentrated along the cell membrane, while the lipid/mucous material remains centrally located, as in sebokeratinocytes of the apteric areas of avian skin. Some lipid and mucus is also incorporated into typical β-cells of the outer scale surface, so that lipids are part of the fully keratinized hard keratin layer of the alligator. Lipids within β-cells of outer scale surfaces and α-cells of the hinge region are probably responsible for limiting water loss and ion movements across the skin. Neither typical mammalian keratohyalin granules nor lepidosaurian keratohyalin-like granules were detected anywhere in alligator epidermis. The combination of anti-β and anti-α keratin antibodies revealed different distributions of β- and α-keratins. In late embryonic stages (25–26 to hatching), β-keratin occurs only in the upper suprabasal cells, in prekeratinized and keratinized layers, whereas α-keratin bundles (tonofilaments) remain only in the lowest layers. The cross-reactivity of the β-antibody, produced against a chick scale keratin, further shows that avian and crocodilian hard (β) keratins share common antigenic sites, reflecting a phylogenetic affinity between these taxa.