Echinoderm Research, De Ridder. Dubois, Lahaye & Jangoux (eds) 0 1990 Balkema, Rotterdam. ISBN 90 6191 141 9 Biological control of skeleton properties in echinoderms Ph. Dubois* Laboratoire de Biologie marine (CP 160), Universitd Libre de Bnucelles, Bnucelles, Belgique ABSTRACT: Echinoderms have a high-magnesium calcite skeleton whose cristallographic, chemical, and morphological properties make it different from abiotic calcites. The available data show that the single crystal behaviour, the conchoidal fracture properties, the high-magnesium content, and the stereom structure could be partly explained by the properties of the organic nacromolecules associated with the mineral phase. Data suzest that these compounds could produce the stereom properties through stereochemical interactions with calcite. of CaC03 and MgC03 in a calcite lattice. i.e. a high-magnesium calcite (Chave Echinoderms have a calcitic skeleton of 1952). The RgC03 content ranges from 3.0 mesodermal origin. This skeleton is to 43.5 mole?/, (Schroeder et al. 1969) developed by most postmetamorphic and averages 13.5-16.2 mole% according individuals as well as by larvae of to the class considered (V!eber 1969). echinoids and ophiuroids. The growing Such a concentration would make evidence indicates that the echinoderm skeleton metastable in postmetamorphic and larval skeletons are standard inorganic temperature/ pressure homologous structures (see Emlet 1985, conditions (Lerman 1965). Parks et al. 1987, Drager et al. 1989, Most single ossicles show properties Dubois 6r Chen 1989, Richardson et al. of light polarization and X-ray 1989). The echinoderm skeleton may be diffraction as though they were carved considered as a paradigm of biologically out of a single crystal of calcite even controlled mineralized structures (I.:ann when they are several centimetres in et al. 1989). differing by most of its length (Raup 1959. Nissen 1969. Donnay & properties from abiotically synthetized Pawson 1959). Furthermore, contrary to calcites. what could have been supposed from these Each skeleton element (the so-called properties of the ossicles, trabeculae ossicle) consists of a rounded do not fracture along the usual tridimensional network of trabeculae , rhombohedra1 cleavage of calcite but the stereom, devoid of any apparent show conchoidal fractures (Nichols & crystalline feature (larval spicules Currey 1968, Nissen 1969). correspond to isolated trabeculae) This paper reviews the present (Nichols & Currey 1968). Furthermore, knowledge of the biological mechanisms the stereom surface has the which control and generate the stereom characteristics of a so-called "periodic properties. minimal surface" (viz. a surface that divides space into two interpenetrating regions, each of them being a single multiply connected domain with no connection with the other) (Donnay & Althou~h a few ossicles were Paason 1969). demonstrated to behave optically as The mineral phase is a solid solution polycrystals (Towe 1967, Donnay & Pawson 1969, M5rkel et al. 1971, MZrkel 19791, *. Senior Research Assistant NFSR most of the echinoderm skeleton is made (Belgium) of phenotypically monocrystalline ossicles. Data are only available on the echinoid larval spicules, and (2) that mechanisms which control the it is possible to cause thcir crystallography of the latter. niisali~nment by inhibition of the The properties characteristic of El-glycosylation of proteins, suggesting single crystals can result frorr! either a direct involvement of the an actual monocrystal i.. a intraspicular glycoproteins in crystal crystalline net1.1ork devoid of alignment. discontinuities) or a highly ordered Conchoidal fractures of stereo= polycrystalline aggregate. In a trabeculae are probably also due to the Siomineralization context, the presence presence of ION. Berman et al. (1988) of coherent organic material within the showed that calcite crystals which were skeleton is a steppine stone in the abiotically grown in presence of S?-: denonstration of the polycrystalline contained specifically adsorbed Sil on nature of the mineral phase (Tovre 1972). their (li00)* planes and cleaved ":vith Contrary to previous statements in the difficulty" along a curved surface of literature, the echinoderm stereon does nlassy appearance reminiscent of the - contain organic macromolecules (see !!ilt conchoidal fracture of echinoderm Pt Denson 1933 and Dubois 3 Chen 1989 for stereom. These authors concluded that, review ) . These we classically separated during cleavage, the SII adsorbed on the into EETA-soluble and EDTA-insoluble TOO) planes creates a- continuous constituents, viz. thc soluble and interference with tile (1014) cleavage insoluble matrix; S:.! and I;>:, planes of calcite resulting in no respectively (the functional validity of well-defined cleavage direction, i.e. in this dichotomy is, however, debated; see s conchoidal fracture. Furthermore, the Yheeler S Sikes 1989). The soluble unusually high strength of the components are principally proteins, echinoderm stereom (Weber et al. 1969, several of them being acidic Enlet 1992) is sugnested to result from PJ-slycosylated slycoproteins (Weiner both the qeneral presence of IOt.1 (makin5 1985, Benson et al. 1986, Di Virgilio h the stereom a composite material) and Dubois unpubl.). Addadi G Weiner (1969) from the specific inclusion of S:l: in proposed that these proteins are cleavage-inhibiting planes. intracrystalline and not structured in a Different treatments of the conchoidal coherent framework. They consequently fractures result in a differential suggested that the intrastereomic etching pattern made of whiskered organic macromolecules (ION) only induce structures parallel to the general local discontinuities in the crystalline c-axis of the ossicle (Pearse & Pearse network, dividing the stereom element 1975. Okazaki G Inou6 1976, Oqrieill into mosaic blocks but not into discrete 1981. Dubois & Jangoux 1985). nicrocrystals (that would conpose a true Differential etching patterns of polycrystalline agzregate). However, biomineralized structures may result Addadi .% 1:leiner (ls89) only considered from selective inclusion of organic th~soluble constituents and overlooked material during mineralization (Addadi & the intrastereomic insoluble Yeincr 1989). Dubois (1988, see also constituents. Since morphological Dubois ,& Chen 1989) suggested effective investigations showed that a coherent involvement of 10:: in the generation of organic material occurs in both larvsl the whiskered structures. He showed that spicules and postmetamorphic ossicles calcite crystals, which were epitaxially (Benson et al. 1933, Dubois 1988), it is grown on fractured trabeculae in most likely that this material presence of S11 in an abiotic system, introduces extensive discontinuities in ta!<e a similar whiskered appearance the crystalline network of calcite, (whereas epitaxial cleavage rhombo- making thus echinoderm ossicles true hedrons were formed in control polycrystalline aggregates composed of experiments 1. Furthermore, Addaci & hifihly ordered microcrystals. This I:.!einer ( 1989) suggested that tine suggests that a biological control whiskered structures observed within the should be exerted over the microcrystal trabeculae are mosaic blocks with IO:.: alignment. A first clue is given by adsorbed on their (hki0) faces. Indeed, Xizoguchi et al. (1981) who reported the abiotically generated whiskered that spicules of echinoid embryos reared structures show morphological in presence of tunicamycin have no extinction position in polarized light. *. !,Tiller-Bravais indices of the TOO) This indicates that (1) discrete and synetry related faces of calcite. microcrystals actually occurs in accordinz to the hexagonal notation. similarities with some calcite calcifying site is either the plasma truncations (Dubois 1988). Whether the membrane or a vacuolar membrane from the whiskered structures correspond to skeleton-forming cells (see Dubois & mosaic blocks or to discrete Chen 1989). This membrane frames the microcrystals is still unclear. However. shape of the calcifying site before their dimensions correspond to the mineralization starts, prefiguring the "sub-micrometre sized" mosaic blocks morphology of the future trabecula (see detected in a stereom element by Blake Wilt 1987 and Dubois .& Chen 1989, for et al. (1984). This suggests that the review). Furthermore, experimental echinoderm stereom could be composed of modifications of the shape of the ordered microcrystals (separated by calcifying vacuole in echinoid larvae . coherent layers of organic material and induce corresponding modifications in revealed by tunicamycin treatment) which the shape of the resulting spicule are subdivided in mosaic blocks (Okazaki 1962). The mechanism by which delimited by intracrystalline SM. the membrane controls stereom morphogenesis is not known. Two processes may be proposed: a spatial confinement of trabecular growth and/or an unspecific inhibition of trabecular Inorganic high-magnesium calcites are growth by organic molecules occurring on metastable in standard temperature/ or in the vicinity of the membrane. pressure conditions (Lerman 1965). MSrkel et al. (1986, 1989) suggested Moreover, in the same environmental that the latter takes place in echinoid conditions, the presence of magnesium tooth calcification. ions in a supersaturated solution of Growth directions of echinoid larval calcium carbonate results in the final spicules (which me homologous to single precipitation of aragonite (Kitano