Studies on the ultrastructure and growth of skeletons of the test and spines in the sea urchin, Strongylocentrotus intermedius エゾバフンウニにおける殻および棘の骨格の成長に関する微細構造学的研究

博士論文

To investigate the growth mechanism of sea urchin dermal skeletons， the morphology and histotopography of cells and their products in intact and regenerating test plates and spines of Strongylocentrotus intermedius were examined by light and electron microscopy (TEM and SEM). 1． Fibrous elements were abundantly distributed at the sutural areas． Their histochemical characteristics showed that they were collagenous fibers combined with acidic and neutral mucopolysaccharides. By TEM observations， the fibers were shown to be composed of thick elements having a typical banding Pattern of mammalian collagen fibers and thin fibrils. The collagenous fibers closely enclosed the trabecular surface. Although these fibers seemed to form a microenvironment for mineralization， they were not a constituent of the calcifying matrix． They may play an inhibitory role in the mineralization to control trabecular growth and shape． 2.Five types of cells were identified in the organic stereom of decalcified test plates． Among them， small basophilic cells having elongated cytoplasmic processes，were identified as sclerocytes． They attached closely to trace holes of decalcified trabeculae． Other cells including spherule cells， eosinophilic cells and vacuolated cells seemed to have no direct relation to the formation of skeletal trabeculae． 3・SEM observations revealed that the sclerocytes were covering closely the trabecular surface with numerous elongated cytoplasmic processes， some of which had bladder-like cytoplasmic knots. This feature was particularly conspicuous at the periphery of growing trabeculae， where a meshwork woven by the processes was closely attached． 4． Flat cells having elongated cytoplasmic processes were observed by SEM to appear around the trabecula surface，dominantly on the portion where an erosion caused an enlargement of the stereomal pore space． The flat cells were conspicuous at the sutural areas of young urchins，plates, Also， there were cells having scrubbed brush－like projections． Together with the flat cells， they were regarded as resorbing cells． 5． Five types of coelomocytes， phagocytic leucocytes, spherule cells， vibratile cells， hyaline cells and reniform cells， were identified in the coelomic fluid of S． Intermedius. At clotting in vitro the phagocytic leucocytes formed a basic framework of envelops derived from their cytoplasmic processes. The framework incorporated other coelomocytes to form a stereom-like structure. The vibratile cells were also observed to participate in the clot formation. By behavioral analogy, the phagocytic leucocytes were considered to migrate into the skeletal tissue as progenitor cells of the sclerocytes. 6. In regenerating test and spines, a calcifying vacuole developed in the “primary” sclerocyte was observed to take part in the initiation of mineralization. A crystal grown in this intracellular vacuole at the primary stage was observed to be exposed to the extracellular space as the crystal grew larger． Further growth of the crystal was continued to be surrounded by syncytial cytoplasmic processes of the ”secondary” sclerocytes， Grown-up trabeculae were directly coated by a continuous membrane sheet or beaded small vesicles， These structures suggested a sequential process of two phases of mineralization， intracellular and extracellular， in the regeneration of test plates and spines． 7． Histological and ultrastructural evidence obtained from the ontogenetic and the regenerating test plates and spines showed that growth of skeletal trabeculae of sea urchins may be carried out by sclerocytes transformed from phagocytic leucocytes which were accumulated at the growing site． The sclerocytes may deposit calcite crystals first in intracellular vacuoles and then liberate them in the extracellular space, where cytoplasmic processes of other sclerocytes participate in further growth of the crystals leading to a mature stereom． The assemblage of the elongated cytoplasmic processes may actively contribute to the crystal growth．