amorphous precursor phases in the sea urchin spicule, one of which is hydrated, and
shows spectra exactly equivalent to those of synthetic hydrated ACC. Another
phase may be anhydrous ACC, but at the time that work was published, there was
no synthetic equivalent to compare the spectra to. The final phase is calcite, as
expected, and its spectra are similar to those of geologic and synthetic calcite (Politi
et al. 2008). A recent study by Radha et al. (2010) showed that the synthetic
anhydrous ACC and spicule ACC have the same enthalpy of transformation into
calcite, thus providing strong evidence that the intermediate phase in spicules is
indeed anhydrous ACC.
Although echinoderm biominerals behave as single crystals of calcite in
polarized light and X-ray diffraction, they do not cleave as crystals. Their fracture
surfaces are conchoidal, with curved surfaces and curved edges reminiscent of
amorphous glasses. Elegant work by Berman et al. gives compelling evidence that
the conchoidal fracture is due to proteins. Calcite rhombohedra were grown in vitro,
in the presence of acidic glycoproteins extracted from sea urchin skeletal elements.
Not only did the synthetic calcite rhombohedra occlude the proteins, but they also
fractured conchoidally (Berman et al. 1988).
This pioneering experiment, and many others that followed it, raise an interesting question: how can a very small amount of organic molecules, e.g., 0.1w% in
spicules, make the fracture surface so macroscopically different from a cleavage
plane of a crystal? We propose that layers (atomic layers in a crystal or sheets of
paper in a sheaf) do not need to be “glued” together extensively. A few spots of
“organic glue” between each pair of subsequent layers are sufficient to keep the
sheaf or the crystal together, thereby preventing cleavage into separate layers.
Hopefully, this hypothesis can be evaluated by obtaining more detailed information
about the exact locations of occluded proteins in the skeletal structures discussed
here.
7.4 Recent Work on the Adult Spine
The iconic feature of sea urchins is their adornment of spines, which can range from
millimeters to scores of centimeters, depending on their location on the test, and on
the species. Spines are long and tapered columns of calcite, ending in a sharp point,
well designed for defense and abrasion. Their spongy texture provides spaces for
several different kinds of dermal cells, and coelomocytes. The surface is covered
with an epithelium, meaning that these hard conspicuously external elements are
truly endoskeletal.
Figure 7.4 shows the mineralized portion of a spine after bleaching and transverse fracture. The young spine is trabecular and fenestrated, as is the forming test
plate. This morphological structure is termed “a stereom.” As the spine matures,
it becomes more and more heavily mineralized, displaying radially arranged sectors
connected by transverse bridges. Sometimes the center is hollow and occupied by
cells, sometimes occupied by both cells and extracellular matrix.
7 Molecular Aspects of Biomineralization of the Echinoderm Endoskeleton
209
shows spectra exactly equivalent to those of synthetic hydrated ACC. Another
phase may be anhydrous ACC, but at the time that work was published, there was
no synthetic equivalent to compare the spectra to. The final phase is calcite, as
expected, and its spectra are similar to those of geologic and synthetic calcite (Politi
et al. 2008). A recent study by Radha et al. (2010) showed that the synthetic
anhydrous ACC and spicule ACC have the same enthalpy of transformation into
calcite, thus providing strong evidence that the intermediate phase in spicules is
indeed anhydrous ACC.
Although echinoderm biominerals behave as single crystals of calcite in
polarized light and X-ray diffraction, they do not cleave as crystals. Their fracture
surfaces are conchoidal, with curved surfaces and curved edges reminiscent of
amorphous glasses. Elegant work by Berman et al. gives compelling evidence that
the conchoidal fracture is due to proteins. Calcite rhombohedra were grown in vitro,
in the presence of acidic glycoproteins extracted from sea urchin skeletal elements.
Not only did the synthetic calcite rhombohedra occlude the proteins, but they also
fractured conchoidally (Berman et al. 1988).
This pioneering experiment, and many others that followed it, raise an interesting question: how can a very small amount of organic molecules, e.g., 0.1w% in
spicules, make the fracture surface so macroscopically different from a cleavage
plane of a crystal? We propose that layers (atomic layers in a crystal or sheets of
paper in a sheaf) do not need to be “glued” together extensively. A few spots of
“organic glue” between each pair of subsequent layers are sufficient to keep the
sheaf or the crystal together, thereby preventing cleavage into separate layers.
Hopefully, this hypothesis can be evaluated by obtaining more detailed information
about the exact locations of occluded proteins in the skeletal structures discussed
here.
7.4 Recent Work on the Adult Spine
The iconic feature of sea urchins is their adornment of spines, which can range from
millimeters to scores of centimeters, depending on their location on the test, and on
the species. Spines are long and tapered columns of calcite, ending in a sharp point,
well designed for defense and abrasion. Their spongy texture provides spaces for
several different kinds of dermal cells, and coelomocytes. The surface is covered
with an epithelium, meaning that these hard conspicuously external elements are
truly endoskeletal.
Figure 7.4 shows the mineralized portion of a spine after bleaching and transverse fracture. The young spine is trabecular and fenestrated, as is the forming test
plate. This morphological structure is termed “a stereom.” As the spine matures,
it becomes more and more heavily mineralized, displaying radially arranged sectors
connected by transverse bridges. Sometimes the center is hollow and occupied by
cells, sometimes occupied by both cells and extracellular matrix.
7 Molecular Aspects of Biomineralization of the Echinoderm Endoskeleton
209