As the primary interface between an organism and the environment, the integument not only provides protection against mechanical insult and physiological stresses, but it also plays obvious roles in communication, locomotion, and external stimulus detection and reception. In addition to numerous specializations at the level of the individual cell, vertebrates have also evolved a wide variety of unique multicellular appendages and organs with diverse structural and topographic organizations, and functional properties. Although ongoing investigations are beginning to cast light on the evolution and development of this crucial organ system, much work remains to be done. This issue of Journal of Anatomy contains contributions from a symposium held at the 8th International Congress of Vertebrate Morphology in Paris, France, in July 2007, entitled ‘The Integument Story: Origins, Evolution and Current Knowledge’. Our goal was to provide a comprehensive and long overdue summary of the evolution, development, structure, and diversity of vertebrate skin and integumentary derivatives. If this symposium was successful, it is due to our many colleagues who participated.
In this issue, the integument story consists of eight review papers (stemming from the Symposium) and two additional and complementary articles. As in the Symposium, we have organized these contributions into mineralized (‘hard’) or keratinized (‘soft’) related topics to draw attention to both these fundamental integumentary components.
The issue begins with two broadly comparative contributions exploring the evolutionary developmental biology (evo-devo) of mineralized integumentary elements. Although highly reduced among most modern taxa, the integumentary skeleton was once the predominant skeletal system, and much of our knowledge of early vertebrates is almost entirely based on fossilized elements of the skin. Sire, Donoghue and Vickaryous begin with a comprehensive review of the integumentary skeletal system from its early origin among 450+ million-year-old stem-gnathostomes, the ‘jawless fossil fish’, to modern sharks and bony fish. Complementing this coverage of aquatic vertebrates, Vickaryous and Sire provide an up-to-date revision of this organ system among tetrapods, with the bulk of the contribution focused on osteoderms. These papers integrate histological and developmental data from extant and fossil taxa within a revised phylogenetic framework, and in doing so provide new scenarios for integumentary skeletal evolution and new hypotheses of skeletal tissue homology.
Teeth are one of most popular subjects in skeletal evo-devo, and receive coverage in three papers. Huysseune, Sire and Witten review the origin of teeth, beginning with a comparison of the two leading theories of dental evolution (‘outside in’, from ectoderm in conjunction with the jaws, or ‘inside out’, from endoderm independent of the jaws). Their findings are intriguing, and provide compelling support for teeth evolving before jaws but from ectoderm (modified ‘outside in’). They go on to discuss the evolution of tooth distribution and molecular regulation of tooth formation in bony fish. Continuing on the subject of teeth, Davit-Béal, Tucker and Sire review tooth and enamel loss in tetrapods. These authors compile the available comparative data, with an initial focus on birds for which molecular data are available. They tentatively trace back the origin of tooth and enamel loss in the various lineages and try to answer the question of how these taxa have survived tooth loss. In the final tooth paper, Caton and Tucker provide a comprehensive review of the current knowledge of tooth development in the mouse. They particularly focus on the genes, genetic pathways and epithelial–mesenchymal interactions that control dentition patterning (the homeobox code), tooth shape and tooth number.
The second part of the issue deals with topics related to the keratinized integument. Bragulla and Homberger set the stage by providing a detailed overview of keratin biology, including extensive coverage of the entire field. Continuing with the keratinized theme, Alibardi, Dalla Valle, Nardi and Toni review the evolution of hard structural proteins (keratin-associated proteins and their genes) in sauropsids (reptiles) and mammals. They suggest that a new class of small matrix proteins might have originated after mutation of an ancestral protein and that the original protein evolved differently in the various reptilian lineages, including birds. The contribution by Dhouailly revisits the origins of feathers in birds and hairs in mammals. This author proposes an exciting and well supported hypothesis in which hairs could have evolved from epidermal glands and feathers from granulated integument. Along the same lines she proposes that the scales on bird feet could be derived from feathers. In addition, regulation of the Wnt/beta-catenin pathway seems to play an important role in keratinized integument evolution.
The last two papers are original contributions investigating the anatomy of claws. First, Maddin, Eckhart, Jaeger, Russell and Ghannadan provide us with previously unknown details of the claw structure of the aquatic frog Xenopus. Their comparisons of claws from Xenopus with those of mice suggest separate origins for these structures in amphibians and mammals. Finally, Homberger, Ham, Ogunbakin, Bonin, Osborn, Hossain, Barnett, Matthews, Butler and Bragulla describe the anatomy of the domestic cat claw, a familiar and yet widely overlooked integumentary appendage. Using an integrated approach including light and scanning electronic microscopy, and computed tomography, these authors provide a thorough (and well-figured) structural investigation.
We appreciate the generous offer from the Anatomical Society of Great Britain and Ireland to publish this Symposium in a special issue of the Journal of Anatomy. We are grateful to the Editors, Professor Gillian Morriss-Kay and Edward Fenton, for their constant support, their patience and their help in our task. We also offer our sincerest thanks to all the authors who accepted our invitation to participate and the many expert reviewers for their insightful comments. Financial support for the Symposium came from COST B23 and the Université Pierre & Marie Curie.