Skeleton resorption in echinoderms: Regression of pedicellarial stalks inSphaerechinus granularis (Echinoida)

Summary

When a globiferous pedicellaria ofSphaerechinus granularis injects its venom, the head autotomizes whereas the stalk remains on the test and enters a regression process with concomitant resorption of its supporting ossicle (i.e. the rod). Scanning electron microscope investigations of the morphological changes undergone by the stereom of resorbing rods show that: (1) resorption proceeds both axially and laterally, and leads to a reduction of approximately 80% of the original length of the rod, (2) secondary growth of new stereom processes occurs concomitantly with resorption but never ensures even a partial regeneration of the rod, and (3) resorption and secondary growth stop before the rod is totally destroyed leaving a static stump that remains in place up to 190 days. Particular resorption figures result from either the axial or the lateral resorption of the rod shaft. These consist chiefly of terraced conical cupules, dense cylinders and concentric lamellae whose walls or edges are typically made of closely piled and/or aligned subprismatic crystallites. Whatever their location along the rod, these crystallites always organize strictly parallel to the rod axis. Whether the crystallites are mosaic blocks composing larger monocrystalline units or discrete monocrystals themselves is for the moment unclear. A growth model, which accounts for the observed resorption figures, is proposed for the shaft of pedicellarial rods. According to this model, the early growth of the shaft would produce elongated, interconnected trabeculae (initial trabeculae) made of densely piled and perfectly aligned crystallites. Thickening and coalescence of adjoining trabeculae would progressively occur by adjunction around the initial trabeculae of successive and concentric layers of similarly arranged crystallites. Coalescent trabeculae would then be cemented together in a perforate stereom layer by the final deposition of larger crystallite layers surrounding the whole shaft periphery. Growth of secondary stereom processes occurs both in the resorbing rod (here the newly formed processes are resorbed soon after they have been produced) and in rods where resorption has stopped. These are always irregular processes that localize near or on the actual sites of resorption. It is suggested these processes result from an uncontrolled activation of the skeleton-forming cells in areas where the concentration of calcium ions increases as a consequence of calcite resorption.