| Cartilage is a type of connective tissue that is often considered restricted to vertebrates, however cartilaginous tissues are also found within invertebrates. Unfortunately, most definitions and classifications of cartilage suffer from a profound vertebrate bias. I find that cartilage is defined most precisely using histological criteria, considering as criteria the composition of the extracellular matrix and the presence of histologically distinct chondrocytes. I have undertaken a rigorous analysis of connective tissues from a number of representative invertebrate lineages using histology, immunochemistry, and developmental data. I find cartilage to be restricted to molluscs, polychaetes, and arthropods, whereas chondroid connective tissues (which have similar histological and structural properties to cartilage) are much more widespread amongst invertebrate lineages. I have expanded the classification system for cartilage to include a variety of vesicular cartilages, thereby accounting for the diversity of histologies exhibited by invertebrate cartilages.; The degree of homology between invertebrate and vertebrate cartilages has been unclear; phylogenetic relationships between metazoan lineages have undergone a series of revisions in recent years, making the evolutionary relationships between cartilage from different lineages difficult to discern from phylogeny alone. I discuss the relationships between cartilage and other related connective tissues using a supertree phylogeny I created from analysis of metazoan molecular systematics, concluding that cartilages from all lineages likely derived independently from a common chondroid connective tissue; thus forming a family of tissues that can be considered homologous at a tissue-level. Although the precise relationships between cartilages within different lineages remains obscured by the imprecision of current metazoan systematics, patterns in the evolution of cartilage as a tissue type can be elucidated through comparisons of histology and development of invertebrate cartilages.; I have identified a tendency within cephalopod molluscs and sabellid polychaetes towards reduction or loss of cartilages that accompanies decreases in body size. Developmental data collected from cephalopods---wherein cartilages are histologically most similar to vertebrate hyaline cartilage---illustrates that cartilage most often differentiates from uncondensed mesenchymal cells near the end of embryonic development. (Abstract shortened by UMI.)... |
