Development and structure of the skin in the Australian lungfish (Neoceratodus forsteri) in relation to epidermal adaptation of tetrapods

Analysis of skin development in the Australian lungfish, Neoceratodus forsteri, indicates that keratinocytes form 2 to 3 irregular layers in tadpoles of 2–5 cm in length and are fully stratified (7–12 layers) in juveniles of 16.5–26 cm in length. The epidermis produces mucus and mainly contains intermediate filament keratins of acidic nature but also a minute amount of neutral-basic keratins (pI 7.0–7.3), which is unique among fish. The latter keratins of 60–62 kDa, are generally present in tetrapods but the specific gene structure and amino sequence of acidic, neutral and basic keratins in N. forsteri are presently unknown. No other epidermal proteins are detected although biochemical data suggest the presence of non-keratin proteins, including a basic type (pI 8.0–8.2) of around 32 kDa of molecular weight. The spinulated bony layer of elasmoid scales contains glycoproteins involved in mineralization, such as osteonectin and osteopontin. Also, alkaline phosphatase, involved in calcium precipitation onto the collagen matrix, has been detected in scales of juveniles up to 26 cm in length. The present review concludes that the knowledge of the data derived from epidermal and dermal protein annotation from the recently sequenced genome of this fish species will contribute to address the study on the evolution of tetrapod skin.