COLE, A.G.; HALL, B.K.; Dalhousie University; Dalhousie University: Evolution of cartilage: What have invertebrates told us?

Cartilage is a tissue that is found throughout the metazoa. We will discuss the histological structure of cartilage and chondroid connective tissues from five different metazoan groups: brachiopods, polychaete annelids, cephalopod mollusks, chelicerate arthropods, and enteropneust hemichordates. From this work we have devised a new view of cartilage as a metazoan connective tissue and will offer new categories for cartilage classification. In contrast to the view of cartilage that is purported by histology textbooks, cartilage is very diverse in histological structure and form. Within vertebrates cartilage varies from being largely matrix dominated (hyaline cartilage) to being highly cellular (cell-rich cartilages). This diversity in structure is paralleled within the invertebrate cartilaginous tissues, highlighting both convergent and parallel evolution of cartilage as a tissue type. Cartilages found in extant taxa – invertebrate and vertebrate – represent derived forms of this metazoan tissue type, thus vertebrate hyaline cartilage should not be viewed as typifying cartilage as a tissue type. The presence of cartilage outside the vertebrates suggests novel interpretations for the evolution of vertebrate skeletal tissues, including that the ability to form cartilage was present before the evolution of vertebrates. Immunoreactivity of some invertebrate cartilages with antibodies that recognise molecules specific to vertebrate bone suggests an intermediate phenotype between vertebrate cartilage and bone. Amassed, this data suggests that both major skeletal tissues in vertebrates – bone and cartilage – are derived from a common chondroid connective tissue, which can be found throughout extant metazoan groups.