Marianne Ghyoot

Fine structure and presumed functions of the pedicellariae of Echinocardium cordatum (Echinodermata, Echinoida)

SummaryTridactylous, trifoliate, and globiferous pedicellariae occur on the body surface of Echinocardium cordatum. Tridactyles have three forms: the typical, the rostrate, and the large forms. Both typical and rostrate tridactyles and trifoliates occur all around the echinoid body (trifoliates are, however, 4 times more numerous than tridactyles). Large tridactylous and globiferous pedicellariae are restricted to the peribuccal area.As a general rule tridactyles and trifoliates are similar in morphology. The distal part of the valves forms an open blade and bears lateral teeth and/or denticles (single or in combs). The stalk consists of a rigid proximal part supported by an axial rod and a flexible distal part which includes an axial fluid-filled cavity. The cavity is surrounded by muscle fibers and acts as an hydroskeleton, allowing the undulating-coiling movements of the flexible part of the stalk. Trifoliates are always active while tridactyles react only to direct or indirect mechanical stimulation.The valves of the globiferous pedicellariae have a tubular distal part whose upper opening is surrounded by teeth. There is no differentiated venom gland but a cluster of epithelial glandular cells located at the level of the valve upper opening. A small ciliary pad occurs just below the glandular cluster. Globiferous stalks are not flexible, being supported for their full length by an axial rod. Globiferous pedicellariae appear to be sensitive only to chemical stimulation.The presumed functions of E. cordatum pedicellariae are (1) cleaning of the body surface and ciliary structures (trifoliates), (2) protection against sedimenting particles (tridactyles), and (3) defense of the peribuccal area against potential small predators (globiferous pedicellariae).

Integumentary resorption and collagen synthesis during regression of headless pedicellariae in Sphaerechinus granularis (Echinodermata: Echinoidea)

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.

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

SummaryWhen 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.

The venom apparatus of the globiferous pedicellariae of the toxopneustid Sphaerechinus granularis (Echinodermata, Echinoida): Fine structure and mechanism of venom discharge

SummaryGlobiferous pedicellariae of Sphaerechinus granularis are venomous defensive appendages consisting of a stalk bearing a head made of three movable jaws. Each jaw is supported by a calcareous valve ending with a terminal grooved tooth. A venom apparatus is located in each jaw and consists of a venom gland surrounded by a muscular envelope and terminating in a duct which completely encircles the terminal tooth of the valve. Contrary to previous statements, the duct does not lie inside the groove of the terminal tooth. In mature pedicellariae, the venom is stored in intracellular vacuoles of highly differentiated cells which are no longer active. The cells fill the whole space of the venom gland which is without a lumen; they are segregated into two types that occur in distinct regions and differ from each other by morphological and staining properties of their secretions. Upon contraction of the muscular envelope, the venom is released via a holocrine mechanism and infiltrates the predators tissues through the wound inflicted by the three calcareous teeth of the valves. In no case is the venom emitted through the tooth groove.