Cartilage differentiation in cephalopod molluscs

Amongst the various metazoan lineages that possess cartilage, tissues most closely resembling vertebrate hyaline cartilage in histological section are those of cephalopod molluscs. Although elements of the adult skeleton have been described, the development of these cartilages has not. Using serial histology of sequential developmental stages of the European cuttlefish, Sepia officinalis, we investigate these skeletal elements and offer the first description of the formation of any cellular invertebrate cartilage. Our data reveal that cuttlefish cartilage most often differentiates from uncondensed mesenchymal cells near the end of embryonic development, but that the earliest-forming cartilages differentiate from a cellular condensation which goes through a protocartilage stage in a manner typical of vertebrate primary cartilage formation. We further investigate the distribution and degree of differentiation of cartilages at the time of hatching in an additional four cephalopod species. We find that the timing of cartilage development varies between elements within a single species, as well as between species. We identify a tendency towards cartilage differentiation from uncondensed connective tissue in elements that form at the end of embryogenesis or after hatching. These data suggest a form of metaplasia from connective tissue is the ancestral mode of cartilage formation in this lineage.