Sea urchins spines

Figure 11 Spongy stereom of Echinoderms (a) Sea urchin plate showing the various stages of the spongy stereom. (b) Starfish in this article, the stereom is less regular, (c) Ophiurid vertebrae. The whole structure is a single crystal of calcite. (d) Conchoidal fracture of a sea urchin spine...

PoLiTi, Y., Arad, T., Klein, E., Weiner, S. Addadi, L. 2004. Sea urchin spine calcite forms via a transient amorphous calcium carbonate phase. Science, 306, 1161-1164. 

Ech inodermata A phylum of marine invertebrates that includes the sea urchins, starfish, brittlestars, and sea cucumbers. Echinoderms have an exoskeleton (test) of calcareous plates embedded in the skin. In many species (e.g. sea urchins) spines protrude from the test. A system of water-filled canals (the water vascular system) provides hydraulic power for thousands of tube feet saclike protrusions of the body wall used for locomotion, feeding, and respiration. Echinoderms have a long history fossils of primi-... 

We have reflected on several issues that are important to fracture characterisation and control in the context of natural self-assembled fibrous materials. To conclude this discussion, it is appropriate to consider some of nature s best examples of optimisation and versatility in fibre-reinforced composites sea cucumber body and the catch ligament associated with the ball-and-socket joint at the base of the sea urchin spine (Fig. 7). These materials are among the living world s oldest fibrous composites, which is testimony to their successful design. They offer several time-tested and thought-provoking lessons for the materials engineer. 

Secretan s syndrome must be differentiated from professional traumatic complaints featuring hard, persistent and spontaneous lymph oedema, which have been described in fishermen, due to various causes and particularly to repeated trauma from sea-urchin spines and the tight cuffs of the wet-suit (Angelini et al. 1990 Angelini and Vena 1991). In these spontaneous, chronic professional cases, lymphography may show alterations of the lymph vessels. Secretan s syndrome must also be differentiated from other types of acute or chronic oedema, such as lymphatic aplasia, recurrent erysipelas, deep thrombophlebitis, angio-oedema, filariasis, venous obstruction, post-surgical disturbances, carcinoma and other tumours of the breast. 

In underwater fishermen, repeated trauma due to penetration of sea urchin spines, together with the constriction of the wrists caused by the wet suit and the low temperature of the water, can induce a different type of delayed reaction, called chronic professional traumatic lymphedema of the hands (Angelini et al. i99oa,b,c). This complaint manifests with hard, persistent edema of the back of the hands and sometimes also of the forearms (Fig. 1). This scleredema persists even many years after abandonment of this working activity and may be accompanied by lymphographic alterations of the limb involved. 

While amorphous precursors were already found and discussed for biomineral systems as a sophisticated way towards minerals as they avoid high salt concentrations with their associated high osmotic pressures, the other mechanisms emerged from purely synthetic systems. Only very recently, the first evidence was reported that, for example, mesocrystals can be found in biominerals like aragonite platelets in nacre [114] or cal-cite nanocrystals in sea urchin spines [116]. Much has still to be done to understand the forces that control the perfect nanoparticle alignment in mesocrystals, as well as the exact building mechanism and the possibilities for manipulating these structiues. However, the toolbox of crystalUzation is clearly extended now. 

The skeleton of sea urchin spines is composed of large single crystals of Mg-rich calcite, which have smooth, continuously curved surfaces and form a three-dimensional fenestrated mineral network. Spines of the echinoids//. trigonariusmAH. mammillatus... 

These can be demonstrated with recreating a mesenchymal stem/progenitor cell niche for regulating cell fate and proliferation. Figure 1.12 shows a foraminifera micro-sphere coated with rat adipose mesenchymal stem cells after 10 days of pellet culture. Figure 1.13 demonstrates mesenchymal stem cells spanning pores of a sea urchin spine. 

Figure 1.13 Mesenchymal stem cells spanning pores of a sea urchin spine.

Vecchio, K.S., Zhang, X., Massie, J.B., Wang, M., Kim, C.W., 2007. Conversion of sea urchin spines to Mg-substituted tricalcium phosphate for bone implants. Acta Biomater. 3, 785-793. 

FIGURE 3 Porous interior of a sea urchin spine calcite single crystal. Polymer coating may serve to prevent fracture from surface damage entrained polymer may enhance the toughness. 

Fig. 8 FETEM images of the oriented nanocrystals with the bridges. (a,b) the nanoscale bridges observed on a sea urchin spine (a) and an eggshell (b), respectively. (c,d) the connected nanocrystals of potassium sulfate (c) and potassium hydrogen phthalate (d), respectively. Reprinted with permission from Wiley-VCH and the Chemical Society of...

Gilbert and coworkers recently reported on the low surface area of biominerals consisting of nanocrystals. Based on the measurements, the surface area of sea urchin spine is comparable to that of spacefilling macroscopic geologic calcite crystals. The mesoerystal struetures of biominerals are different from those of synthetic materials. They suggested that the low specific surface area is ascribed to the presence of ACC in the nanospace. In fact, organic molecules can be introduced in the nanospace from the solution or the liquid state. However, the gas for the measurement of the surface area is not introduced in the nanospace. The structures of the nanospace in mesocrystals remain unresolved problems. 

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