anhydrous ACC and eventually to calcite. Likewise, the adult tooth shows a similar
transition from hydrated ACC to anhydrous ACC to calcite during its formation,
and a similar transition is likely occurring during adult spine regeneration.
We speculate that: (1) the ACC precursor is a general strategy employed in
biomineralization in echinoderms, (2) the numerous occluded proteins play a role
in post-secretion formation of the mature biomineralized structure, and (3) proteins
with “multi-valent” intrinsically disordered domains are important for formation of
occluded matrix structures, and regulation of crucial matrix–mineral interactions,
such as ACC to calcite transitions and polymorph selection.
7.1 Introduction
There has been substantial interest in the echinoderm skeleton, especially that of sea
urchins, for centuries. Skeletal elements of echinoderms are abundant in the fossil
record and, hence, important in paleontology and evolutionary studies. The
enveloping shell-like surface, the test, is often admired for its decorative beauty,
and for its toughness and strength, which are very different from pure calcite.
The phylum Echinodermata is comprised of five extant classes: sea urchins and
sand dollars (echinoidea), sea lilies (crinoidea), sea stars (asteroidea), brittle stars
(ophiuroidea), and sea cucumbers (holothuroidea). This phylum and the phylum
Hemichordata are the closest relatives of the chordates, which include the
vertebrates. Members of these three phyla are the only animals with deuterostome
mode of embryonic development, i.e., the site of gastrulation becomes the anal
pore of the larva or adult. The echinoderms have a hard, mineralized endoskeleton,
which is a composite of calcite and an organic matrix, while the vertebrate
endoskeleton uses calcium phosphate.
The sea urchin embryo is well suited for biochemical and molecular studies, and
identification and characterization of biomineralization proteins have steadily
increased in recent years. There is also intense interest in the mineral phase of
skeletal elements in both larva and adult, as well as in the relationships of organic
matrix and mineral, and biomechanical properties of the skeletal elements. We shall
concentrate in this review on a discussion of recent work on matrix proteins, on
relationships of matrix to mineral, and on recent progress in understanding the
crystal orientation in skeletal elements, and the mineral precursor phases.
It is not feasible to cite or discuss all the important and pioneering studies on
these subjects. Much of the older literature is thoughtfully discussed in books by
Simkiss and Wilbur (1989), and by Lowenstam and Weiner (1989). The chapter by
Raup (1966) on the echinoderm skeleton is also useful. Work on biomineralization
in classes other than echinoids is sparse, and is discussed in a review by Wilt et al.
(2003). Decker and Lennarz (1988) have surveyed earlier studies of spicule formation, and Wilt and Ettensohn (2007), and Wilt and Killian (2008) have published
reviews of the recent findings on the genes that encode proteins associated with
biomineralization in sea urchins.
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P.U.P.A Gilbert and F.H. Wilt
transition from hydrated ACC to anhydrous ACC to calcite during its formation,
and a similar transition is likely occurring during adult spine regeneration.
We speculate that: (1) the ACC precursor is a general strategy employed in
biomineralization in echinoderms, (2) the numerous occluded proteins play a role
in post-secretion formation of the mature biomineralized structure, and (3) proteins
with “multi-valent” intrinsically disordered domains are important for formation of
occluded matrix structures, and regulation of crucial matrix–mineral interactions,
such as ACC to calcite transitions and polymorph selection.
7.1 Introduction
There has been substantial interest in the echinoderm skeleton, especially that of sea
urchins, for centuries. Skeletal elements of echinoderms are abundant in the fossil
record and, hence, important in paleontology and evolutionary studies. The
enveloping shell-like surface, the test, is often admired for its decorative beauty,
and for its toughness and strength, which are very different from pure calcite.
The phylum Echinodermata is comprised of five extant classes: sea urchins and
sand dollars (echinoidea), sea lilies (crinoidea), sea stars (asteroidea), brittle stars
(ophiuroidea), and sea cucumbers (holothuroidea). This phylum and the phylum
Hemichordata are the closest relatives of the chordates, which include the
vertebrates. Members of these three phyla are the only animals with deuterostome
mode of embryonic development, i.e., the site of gastrulation becomes the anal
pore of the larva or adult. The echinoderms have a hard, mineralized endoskeleton,
which is a composite of calcite and an organic matrix, while the vertebrate
endoskeleton uses calcium phosphate.
The sea urchin embryo is well suited for biochemical and molecular studies, and
identification and characterization of biomineralization proteins have steadily
increased in recent years. There is also intense interest in the mineral phase of
skeletal elements in both larva and adult, as well as in the relationships of organic
matrix and mineral, and biomechanical properties of the skeletal elements. We shall
concentrate in this review on a discussion of recent work on matrix proteins, on
relationships of matrix to mineral, and on recent progress in understanding the
crystal orientation in skeletal elements, and the mineral precursor phases.
It is not feasible to cite or discuss all the important and pioneering studies on
these subjects. Much of the older literature is thoughtfully discussed in books by
Simkiss and Wilbur (1989), and by Lowenstam and Weiner (1989). The chapter by
Raup (1966) on the echinoderm skeleton is also useful. Work on biomineralization
in classes other than echinoids is sparse, and is discussed in a review by Wilt et al.
(2003). Decker and Lennarz (1988) have surveyed earlier studies of spicule formation, and Wilt and Ettensohn (2007), and Wilt and Killian (2008) have published
reviews of the recent findings on the genes that encode proteins associated with
biomineralization in sea urchins.
200
P.U.P.A Gilbert and F.H. Wilt