REGULAR ARTICLE Molecular, cellular and histological changes in skin from a larval to an adult phenotype during bony fish metamorphosis Marco A. Campinho & Nádia Silva & Glen E. Sweeney & Deborah M. Power Received: 10 March 2006 / Accepted: 31 May 2006 / Published online: 7 October 2006 # Springer-Verlag 2006 Abstract Developmental models for skin exist in terrestrial and amphibious vertebrates but there is a lack of informa- tion in aquatic vertebrates. We have analysed skin epider- mal development of a bony fish (teleost), the most successful group of extant vertebrates. A specific epidermal type I keratin cDNA (hhKer1), which may be a bony-fish- specific adaptation associated with the divergence of skin development (scale formation) compared with other verte- brates, has been cloned and characterized. The expression of hhKer1 and collagen 1α1 in skin taken together with the presence or absence of keratin bundle-like structures have made it possible to distinguish between larval and adult epidermal cells during skin development. The use of a flatfish with a well-defined larval to juvenile transition as a model of skin development has revealed that epidermal larval basal cells differentiate directly to epidermal adult basal cells at the climax of metamorphosis. Moreover, hhKer1 expression is downregulated at the climax of metamorphosis and is inversely correlated with increasing thyroxin levels. We suggest that, whereas early mechanisms of skin development between aquatic and terrestrial vertebrates are conserved, later mechanisms diverge. Keywords Epidermal type I keratin . Metamorphosis . Thyroid hormones . Skin basal epidermal cells . Halibut, Hippoglossus hippoglossus (Teleostei) Introduction The skin is a complex epithelium and the most extensive interface between the animal and its external environment. In teleosts, skin houses sensory organs essential for survival and helps maintain body shape and protect against shock and infection. From an ontogenic perspective, the epidermis is the major component of teleost embryonic and larval skin, the dermis is present as an acellular collagen lamella and the hypodermis is completely absent (Roberts et al. 1973; Ottensen and Olafsen 1997; Murray et al. 2003; Le Guellec et al. 2004). As teleosts undergo metamorphosis, skin differentiates into the adult structure and, in common with the skin of other vertebrates, is composed of the outermost epidermal layer, the dermis and the innermost hypodermis (Roberts et al. 1973; Ottensen and Olafsen 1997; Murray et al. 2003; Le Guellec et al. 2004). Little is however known about the cellular and molecular events that accompany larval to adult skin differentiation in teleosts. Pleuronectiformes represent an interesting group of teleosts in which to study metamorphosis, as they undergo a dramatic morphological reorganization during this process and change from a symmetrical larva to an asymmetric Cell Tissue Res (2007) 327:267–284 DOI 10.1007/s00441-006-0262-9 This work was carried out within the project “Arrested development: The Molecular and Endocrine Basis of Flatfish Metamorphosis” (Q5RS-2002-01192) with financial support from the Commission of the European Communities. It does not necessarily reflect the Commission’ s views and in no way anticipates its future policy in this area. This project was further supported by Pluriannual funding to CCMAR by the Portuguese Science and Technology Council. M.A. Campinho was sponsored by the Portuguese Ministry of Science (grant no. SFRH/BD/6133/2001). M. A. Campinho : N. Silva : D. M. Power (*) Comparative Molecular Endocrinology Group, Marine Science Centre (CCMAR), Universidade do Algarve, 8005-139 Faro, Portugal e-mail: dpower@ualg.pt M. A. Campinho : G. E. Sweeney School of Biosciences, Cardiff University, Museum Avenue, CF10 3US Cardiff, UK