Endoskeleton/Exo (dermal) skeleton - Mesoderm/Neural Crest: Two pair of problems and a shifting paradigm

Abstract

Summary Vertebrates have both more than one skeleton and multiple skeletal systems reflecting origins from two germ layers (mesoderm and neural crest), locations of skeletal elements as external or internal (exo- and endoskeleton), and separate craniofacial, axial and appendicular skeleton systems. Current studies reinforce the exoskeleton as consisting of bone and dentine to the exclusion of cartilage. The endoskeleton is based in cartilage, which, depending on lineage may be replaced by bone. Current understanding of the nature of the exo- and endoskeletons, modes of ossification (intramembranous, endochondral, perichondral), and the contributions of mesodermal and neural crest cells to the skeleton(s) is summarized and then discussed in more depth using fin rays in paired and unpaired fins as examples. Recent studies demonstrating that fin rays and scales are mesodermal and not neural crest in origin are reviewed and whether trunk neural crest cells have skeletogenic potential is considered. These results are discussed in the context of whether scales and fin rays should continue to be regarded as components of the exoskeleton. The conclusion is that they should, thereby separating germ layer of origin from classification of skeletal elements as exo- or endoskeletal; germ layer of origin does not provide unambiguous evidence for classification of elements as parts of the exo- or endoskeletons. Recent studies indicating that turtle shells are mesodermal in origin provides further evidence of the contribution of mesoderm to skeletal elements traditionally regarded as part of the exoskeleton. Therefore, exoskeleton is not synonymous with neural crest origin. These studies on fish fin rays/scales and turtle shells herald a new paradigm in which germ layer of origin (mesoderm or neural crest) does not equate with classification of elements as parts of the endo- or exoskeleton.