Sea urchins spicules

Volkmer et al. used amphiphilic cyclic peptides based on asparagines and phenylalanine. These peptides favor the formation of calcite crystals. Furthermore, in addition to the normal [10.4] faces, a new set of diamond-shaped [01.2] crystal faces formed, depending on the concentration of the amphiphilic peptides. These additional crystal faces also form in the presence of proteins isolated from sea urchin spicules or sponge spicules [169]. There, similar calcite crystals were formed with [01.1] (/= 1-5) faces [166]. 

Among the organs of living things consisting of calcite, the spicules of sea urchins and the exo-skeleton of coccolithophores attract particular interest. A sea... 

Sea urchin larvae are about a hundred micrometers in diameter. They have an internal skeleton that supports the soft tissues. The skeleton is composed of one or several pairs of intricately shaped spicules, the morphologies of which vary among different species. The spicule is composed of two different minerals, amorphous calcium carbonate and calcite [66]. The mineral phases in the adult skeleton are thought to be similar to those in the larval skeleton [67]. In fact almost the whole echinoderm phylum appears to use this type of material for constructing a large variety of skeletal elements. 

In essence, sea urchin larval spicule formation takes place in a preformed membrane framework that continuously changes. There is, however, also an organic matrix-like framework within the spiculogenic cavity. It is composed of polysaccharides and proteins that remain insoluble after the mineral phase of the mature spicule is dissolved [75]. The framework forms concentric sheaths around the spicule long axis, and has radiating fibers that connect the sheaths laterally. It is not, however, known whether this matrix is preformed and functions as a framework to guide the mineral deposition, or whether it is deposited periodically as the mineral is introduced. 

Figure 1.10. Larval spicules from the sea urchin Paracentrotus lividus at two stages of development, (a) Triradiate stage. The initially deposited single crystal is still visible in the center of the spicule. The direction of the crystallographic a-axes of calcite are marked, (b) Fully developed spicule. The direction of the crystallographic c-axis of calcite is marked by an arrow. The initially formed triradiate part can be seen...

The chiton tooth, dentin and the sea urchin larval spicule reflect the enormous diversity of the field of biomineralization. They differ with respect to the nature of their mineral and macromolecular components, as well as their structures. Few underlying common strategies can be recognized the delineation of a dedicated space in which the mineralized tissue forms, the formation of mineral in a preformed framework within this space, and the precipitation of mineral from a supersaturated phase. In this section we will reexamine some of these underlying issues, focussing in particular on the microenvironment in which mineralization occurs. 

In the chiton tooth, the organic framework components are synthesized and secreted by the cells into the extracellular space, and there they self assemble. By the time mineralization is about to occur the cells are tens of micrometers away from many of the mineralization sites. They must therefore operate by remote control. The mineralization sites themselves are within a complex chitin framework, the dimensions of which are in the nanometer range. The sea urchin larval spicule represents the exact opposite situation. Mineralization occurs in a vacuole defined by a membrane, and the entire apparatus is within a consortium of fused cells (the syncytium). The membrane of the syncytium tightly surrounds the growing spicule [74], Therefore, it has been proposed that the cells directly control spicule formation. The mineralization vacuole is subdivided by framework proteins. Nothing is known about the structure of the one nucleation site per spicule in the larvae, but in the adult a well-defined location, enclosed within a framework, has been identified as the nucleation site [83]. Dentin formation is intermediate between the two. It is an extracellular process, and the distances between cells or cell processes and mineralization sites are in the range of tens of micrometers or several micrometers respectively. Nucleation occurs within the fibril or at its surface and is associated with a site on the fibril surface some 7 or 8 nm wide [54]. The space available for crystal growth within the fibril is even smaller in one of the dimensions, namely 2 or 3 nm wide. 

Studies of the mineralization of the larvae of sea urchins (they can be conveniently grown in synchronous culture) have made it possible to utilize molecular biologic techniques to determine gene expression, protein synthesis, and macro-molecular organization as well as the sequences of mineral deposition (Benson et al., 1987). These are some of the earliest forms to be studied and illustrate the connections between genetics and biomineralization. Stem cells responsible for spicule formation can be isolated and produce normal spicules in vitro (Kitajima and Okazaki, 1980) since spines regenerate the secondary mineralization process can also be studied (Ebert, 1967). 

Spicule formation in the larval stage of the sea urchin commences when mesenchyme cells migrate into special locations where they fuse forming a syncytium that produces a membrane-bound vacuole. The spicule morphology is directed by the size and orientation of the vacuole, which aggregate and connect through stalks... 

Benson S. C., et al. (1987) Lineage specific gene encoding a major matrix protein of the sea urchin emrbyo spicule ... 

Kitaj ima T. and Okazaki K. (1980) Spicule formation in vitro by the descendents of precocious micromere formed at the 8-cell stage of sea urchin embryo. Dev. Growth Differ. 22, 266-279. 

Figure 4. Characteristic fragment of a spicule of a sea-urchin beside colorless quartzes, brown biotites and yellowish rounded carboneous fragments in sherd SB 30 from Genoa, Italy. 1 Nicol. Length of the image area is 0.82 mm.

Y. Kitazume, G. Tamura, A. Kobata, Abnormal spicule formation induced by tunicamycin in the early development of the sea-urchin embryo, Cell Struc. Fund. 1981, 6, 341-346. 

Raz et al. (2003) reported the transient phase of amorphous calcium carbonate in sea urchin larval spicules and the involvement of proteins and magnesium ions in its formation and stabilization. 

Beniash, E., Addadi, L., and Weiner, S. (1999). Cellular control over spicule formation in sea urchin embryos a structural approach, J. Struct. Biol. 125,50-62. 

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