Structural and Functional Elements of the Epidermal Appendages in Birds and Reptiles: Conformations and Modes of Assembly of the Constituent β-Filaments and Keratin Intermediate Filaments (IF).

Much is now known about the structures of the β-filaments and the intermediate filaments that together constitute the bulk of the avian and reptilian (sauropsid) appendages (claws, scales, feathers and beaks). New sequence data from the Rhynchocephalia (the tuatara), the last branch of the phylogenetic classification of the sauropsids to be studied, has confirmed that all members of the sauropsids are based on common structures. In addition, an examination of the sequence data has revealed that the β-filaments in the lepidosaurs (lizards, snakes and tuatara) contain a chain that is likely to be a structural component of two separate filaments, thereby providing a unique feature that could facilitate ordered filament aggregation. Similarly, a Type II IF chain (K80) in lizards appears capable of forming an interaction in inter-filament space that would link adjacent IF through tail-tail interactions. A Type II IF chain in zebra finch (K78LT) also seems likely to play a similar role. More details of the surface lattice structure in the IF have also been obtained, as has information on the lateral packing of the protofilaments in the IF. Consequently, an increasingly detailed picture has emerged of the structure and assembly of the filamentous structures that comprise the corneous appendages in the sauropsids.