PMC-specific gene expression has also been analyzed by analysis of ESTs of a
PMC cDNA library (Zhu et al. 2001). They identified a transmembrane protein,
P16, that is essential for spicule formation, as judged by gene knock-down
experiments (Cheers and Ettensohn 2005). It is a straightforward matter to use
new, powerful methods of protein identification to obtain a complete list of
occluded proteins in the spicule, which has been done for the sea urchin tooth
and spine (Mann et al. 2008a, b; Mann et al. 2010).
7.3 ACC: Discovery, Importance, and Implications
in Other Systems
A resurgence of interest in amorphous minerals was motivated by the discovery by
Beniash et al. (1997) that spicules isolated from sea urchin embryos, especially
those from earlier stages (e.g., prism) prior to the mature pluteus larva, have
substantial amounts of amorphous calcium carbonate (ACC) as identified by
Fourier Transform InfraRed (FTIR) spectroscopy. This has been confirmed by a
variety of other physical techniques, including visible light polarization and X-ray
absorption near-edge structure (XANES) spectroscopy (Politi et al. 2006, 2008).
Though stable forms of ACC containing equimolar amounts of hydration water and
CaCO 3 are known in ascidians, crustaceans, and other animals and plants,
(Lowenstam and Weiner 1989), the ACC of the sea urchin embryo slowly
transforms to calcite, so that the developing larva (a day or two after attaining the
pluteus form) has little ACC. Furthermore, isolated spicules that are stored at
À20
C still slowly transform ACC to calcite (Beniash et al. 1997). Synthetic
ACC prepared in the laboratory is unstable and quickly transforms to the most
stable polymorph, calcite, in minutes, not hours or days.
The ACC found in spicules is apparently not an isolated example. The presence
of amorphous precursor minerals was also observed in regenerating spines of sea
urchins (Politi et al. 2004), in the forming end of the sea urchin tooth (Killian et al.
2009), in continuously growing fin rays of fish (Mahamid et al., 2008), and in
forming tooth enamel from mouse incisors (Beniash et al. 2009). ACC has also been
implicated in the formation of mollusk shells (Weiss et al. 2002; Nassif et al. 2005),
although a recent report did not find ACC in newly deposited nacre (Kudo et al.
2010). An obvious implication of these diverse findings is that formation of an
amorphous phase of the mineral, as a metastable precursor to calcite, aragonite, or
carbonated apatite, might be a general tactic used in biomineralization. We shall
consider that proposition near the end of this review.
Possible atomic structures of synthetically produced ACC have been
investigated using x-ray scattering (Michel et al. 2008; Goodwin et al. 2010), but
heretofore the ACC structure has not been investigated in biomineral amorphous
precursors. The mode of crystal formation and propagation has been analyzed by
Politi et al. (2008) and by Killian et al. (2009). Politi et al. found that there are two
208
P.U.P.A Gilbert and F.H. Wilt
PMC cDNA library (Zhu et al. 2001). They identified a transmembrane protein,
P16, that is essential for spicule formation, as judged by gene knock-down
experiments (Cheers and Ettensohn 2005). It is a straightforward matter to use
new, powerful methods of protein identification to obtain a complete list of
occluded proteins in the spicule, which has been done for the sea urchin tooth
and spine (Mann et al. 2008a, b; Mann et al. 2010).
7.3 ACC: Discovery, Importance, and Implications
in Other Systems
A resurgence of interest in amorphous minerals was motivated by the discovery by
Beniash et al. (1997) that spicules isolated from sea urchin embryos, especially
those from earlier stages (e.g., prism) prior to the mature pluteus larva, have
substantial amounts of amorphous calcium carbonate (ACC) as identified by
Fourier Transform InfraRed (FTIR) spectroscopy. This has been confirmed by a
variety of other physical techniques, including visible light polarization and X-ray
absorption near-edge structure (XANES) spectroscopy (Politi et al. 2006, 2008).
Though stable forms of ACC containing equimolar amounts of hydration water and
CaCO 3 are known in ascidians, crustaceans, and other animals and plants,
(Lowenstam and Weiner 1989), the ACC of the sea urchin embryo slowly
transforms to calcite, so that the developing larva (a day or two after attaining the
pluteus form) has little ACC. Furthermore, isolated spicules that are stored at
À20
C still slowly transform ACC to calcite (Beniash et al. 1997). Synthetic
ACC prepared in the laboratory is unstable and quickly transforms to the most
stable polymorph, calcite, in minutes, not hours or days.
The ACC found in spicules is apparently not an isolated example. The presence
of amorphous precursor minerals was also observed in regenerating spines of sea
urchins (Politi et al. 2004), in the forming end of the sea urchin tooth (Killian et al.
2009), in continuously growing fin rays of fish (Mahamid et al., 2008), and in
forming tooth enamel from mouse incisors (Beniash et al. 2009). ACC has also been
implicated in the formation of mollusk shells (Weiss et al. 2002; Nassif et al. 2005),
although a recent report did not find ACC in newly deposited nacre (Kudo et al.
2010). An obvious implication of these diverse findings is that formation of an
amorphous phase of the mineral, as a metastable precursor to calcite, aragonite, or
carbonated apatite, might be a general tactic used in biomineralization. We shall
consider that proposition near the end of this review.
Possible atomic structures of synthetically produced ACC have been
investigated using x-ray scattering (Michel et al. 2008; Goodwin et al. 2010), but
heretofore the ACC structure has not been investigated in biomineral amorphous
precursors. The mode of crystal formation and propagation has been analyzed by
Politi et al. (2008) and by Killian et al. (2009). Politi et al. found that there are two
208
P.U.P.A Gilbert and F.H. Wilt