Comparative morphology of echinoderm calcified tissues: Histology and ultrastructure of ophiuroid scales (Echinodermata,Ophiuroida)

Summary The calcified body wall of an ophiuroid was investigated by a new method and compared with that of other echinoderms. The previous opinion that the epidermis of ophiuroid arm shields consists of a reduced syncytium continuous with the underlying dermis is incorrect. The epidermis is distinctly separated from the dermis by a basal layer and consists of (1) supporting cells which bear the cuticle, (2) ciliated cells (hitherto unknown and probably sensory), (3) gland cells, and (4) nerve cells with the basal nerve plexus. The overall structure of the epidermis is a three-dimensional tube system (marked by the basal lamina) which penetrates the dermal tissue of the scale's pore space and continues with nerve cords situated below the scale. This arrangement is unique in echinoderms.The dermal sclerocytes largely conform with those of the echinoid Eucidaris. The mineral skeleton is produced intracellularly or intrasyncytially. Moreover, dermal sclerocytes were found to release extracellular microfibrils which have nothing to do with calcite deposition. The attachment of the cuticle to the dermis is achieved by means of epidermal coupling areas. Collagen fibers fasten the scale to the underlying connective tissue sheath. The supposed fibrocytes within this sheath resemble sclerocytes. Each collagen bundle is provided with a strand of nerve fibers which, in contrast to the basal nerve plexus, are naked. They are said to infuence the mechanical properties of the connective tissue.Structures associated with cilia occur in cell types which normally lack a cilium. This finding suggests that most echinoderm cells are potentially monociliate.Abbreviations A apical shield - asp secretory products - B bacteria - bb basal body - bl basal lamina - C ciliated cell - ca coupling area - ci cilium, - cf collagen fibrils - cs cell surface - CTS connective tissue sheath - cu _i inner cuticular layer - cu _m middle cuticular layer - dp distal processes (Sc) - EC epineural canal - G Golgi complex - gv granular vesicle - H haemal vessel - hb homogeneous body - hl horizontal lamina (Su) - j cell junction - L lateral shield - le boundary layer (Sc) - lo distal lobe (Su) - M intervertebral muscle or its attachment - m mitochondrium - mf microfibrils - mu mucus - mv microvilli - mvb multivesicular body - N nerve cell - n nucleus - nf neurofibrils - ng neurogranules - nn naked neurofibrils - O oral shield - P tube foot - Pc phagocyte - pg pigment granules - rl rootlet - RN radial nerve - RV radial vessel - Sc sclerocyte - sh cytoplasmic sheath (Sc) - sj septate junction - Su supporting cell - sv secretory vesicle - T calcite trabeculum - V vertebral ossicle - v vesicle (Su)