Comparative morphology of muscle-skeleton attachments in the Echinodermata

Summary Muscles are generally attached to the skeleton by interconnecting tendons. Each tendon necessarily has a junction with the muscle and another with the skeleton. The ultrastructure of the skeleton is identical in all echinoderm classes. Nevertheless, we found three different types of muscle-skeleton junctions. (1) In Crinoida the muscles are attached almost directly to the calcite trabeculae. (2) Asteroida and Ophiuroida have tendons that arise from the basal laminae of the muscle bundles. They consist of unstriated microfibrils that are attached on the muscle side to electron-dense areas below the sarcolemma of the fingershaped muscle ends. On the skeleton side they embrace the outermost calcite trabeculae. (3) In Echinoida the strong muscles are joined to the skeleton by means of composite tendons. They consist of unstriated tendon cords that adhere to the muscles and of bundles of striated fibrils coiled around the calcite trabeculae. Both kinds of tendons are interconnected in the same way as the links of a chain. Composite tendons are found in junctions that are exposed to multidirectional stress. In Holothuroida there are no true muscle-skeleton junctions and the muscles are apposed to connective tissue.The muscle-tendon junctions in Echinodermata differ fundamentally from the junctions in the protostome Arthropoda or Mollusca, but they were found to be very similar in structure to the muscle-tendon junctions in Vertebrata. This coincidence may refer to a phylogenetic relationship of the two deuterostome phyla. But the tendon-skeleton junctions of the two phyla are dissimilar, for Echinodermata and Vertebrata differ fundamentally in their skeletons. Vertebrate bone consists of extracellular fibrils combined with minute crystals of hydroxylapatite. Echinoderm ossicles are intercellular in origin. They are nothing but the calcified vacuolar system of syncytial sclerocytes, and extracellular fibrils never enter the mineral phase.Abbreviations bl basal lamina - c calcite trabecula - dp distal processes of sclerocytes - el electron-dense layer - m muscle - sf striated tendon fibrils - uf unstriated tendon fibrils - tc trabecle coat