Review. Cell Biology and Immunohistochemical Evidence of the Transition Between an Aquatic to a Terrestrial‐Waterproof Epidermis in the Archosaurian Alligator

The present review summarizes studies on the morphology and cell biology of the developing epidermis in the alligator as an example of archosaurian reptile. From an initial, two-layered epidermis, numerous suprabasal keratinocytes are produced at late stages of embryonic development, some days before hatching. The soft embryonic epidermis contains mucous granules, cysteine-poor IF-keratins and trichohyalin-like proteins produced from genes located in different chromosomal loci, including the EDC (Epidermal Differentiation Complex). These proteins contribute to the mild keratinization of the embryonic epidermis, resembling that of amphibians. The embryonic epidermis is composed from 3 to 4 layers of epidermal layers that are shed before hatching. The 3rd or 4th layer of the embryonic epidermis begins to accumulate Corneous Beta Proteins (CBPs), marking the transition into the definitive corneous epidermis with the production of spindle-shaped beta-cells. The latter accumulate large amount of CBPs, some lipids droplets and, in some areas also melanosomes, giving rise to a relatively hard and impermeable stratum corneous with a patterned pigmentation. The presence of Sox-oxidase in differentiating and maturing beta-cells, catalyzes the formation of numerous disulphide bonds, likely binding IFKs and CBPs in addition to their electrostatic interactions. Intra- and inter-molecular bonds contribute to hardening the corneous material, forming the definitive corneous layer. The latter, with the incorporated lipids allows to post-hatching, juveniles and adult alligators to withstand the freshwater contact and also the dry conditions of the terrestrial environment where they live. The evolution of numerous EDC proteins and CBPs mixed with lipids confers specific adaptive characteristics to the skin in this reptile.