The fine structure and development of calcified skeletal elements in the body wall of holothurian echinoderms

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Abstract
The calcareous ossicles and iron‐rich calcified granules in the integument of sea cucumbers (Echinodermata: Holothuroida) have been examined by light and electron microscopy. Most ossicles are perforated, platelike structures that measure 100–500 μm long and consist of magnesium‐rich calcite. The formation of ossicles occurs within multinucleated syncytia of sclerocytes that are situated in the dermal layer of the body wall. Sclerocytes possess fairly large numbers of mitochondria and are characteristically enveloped by an external lamina. Each ossicle arises as a minute rodlike spicule that branches and develops into a fenestrated skeletal element. When viewed by SEM, fully developed ossicles appear non‐crystalline. Following decoration with synthetically grown calcite seeds, however, each ossicle reveals an ordered array of crystallites that seem to be aligned perpendicular to the ossicle's original plane of growth. Examinations of sectioned ossicles indicate that the mineral phase in each skeletal element lacks a well developed matrix of EDTA‐insoluble organic substances. Ossicles first arise in the ciliated, lecithotrophic larvae of Eupentacta quinquesimita at about 1 week postfertilization. Two‐week‐old specimens adopt an epibenthic existence and subsequently become enveloped by imbricated ossicles that are secreted by sclerocytes resembling those found in adults. In molpadiid holothurians, the adult body wall contains numerous reddish‐brown granules that are chiefly composed of iron‐rich subunits. The dermal granules differ from calcitic ossicles in developing extracellularly in association with finely filamentous material.