On the development and evolution of cartilage within non-chordate metazoa.

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.