ADVANCEMENT MECHANICS OF GROWING TEETH IN SAND DOLLARS (ECHINODERMATA, ECHINOIDEA) - A ROLE FOR MUTABLE COLLAGENOUS TISSUE

Regulation of growth involves the integration of several body systems including nerves, muscles and connective tissues. We demonstrate how c hanges in material properties of a connective tissue permit advancemen t of the continuously growing teeth of sand dollars. During growth, ea ch tooth advances in a tooth slide. During chewing, however, teeth are rigidly attached by collagenous dental ligaments. We found that there was a natural, bimodal variation in tooth looseness where some sand d ollars had teeth so loosely attached that they could not crush sand pa rticles without detaching their teeth. We also found that soaking thes e dental ligaments in divalent cation-free artificial seawater caused more rapid force-relaxation than control artificial seawater. These re sults suggest that the dental ligaments are a special mutable collagen ous tissue (MCT), found in echinoderms, and that sand dollars periodic ally loosen their teeth via changes in the MCT to allow the teeth to a dvance. This process could be under nervous control, as material prope rties of MCT can be altered via nervous control. Thus mutable collagen ous tissue in echinoderms is used not only for many skeletal functions , but also for regulation of tooth growth.