Ultrastructure of the resorption of integumentary tissues (ligaments, muscles, fibrous tissue, nerves, and skeleton) and the synthesis of collagen is described for the first time in echinoderms. Resorption is cell-mediated. Phagocytic cells are characterized by Golgi-derived putative primary lysosomes. Numerous secondary lysosomes and residual bodies occur in the bodies and processes of phagocytic cells. They engulf whole muscle cells and nerve fibres, as well as collagen fibril segments that exceed 1.5 μm in length. Skeletoclastic cells resemble vertebrate osteoclasts, showing a ruffled border, lysosomes, and numerous mitochondria. They surround trabeculae with thick processes to delimit a tubular resorption site. Collagen synthesis occurs in the space formerly occupied by resorbed tissues. Synthesis is performed by fibroblastic cells containing organelles typical of vertebrate fibroblasts, namely distended cisternae of rough endoplasmic reticulum, Golgi cisternae with distended edges, and procollagen granules. Procollagen granules are apparently exocytosed directly to the extracellular matrix. Evidence indicates that resorbing (phagocytic and skeletoclastic) cells and fibroblastic cells may belong to a common phagocyte lineage. These cells share the ability to form elaborate processes and to become syncytial, and their nuclei exhibit iron-containing crystals.
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