membrane and a cytoplasmic sheath without any intervening space, as was shown
by Beniash et al. (1997).
7.2.4 Formation of Postembryonic Skeletal Elements
Larvae that survive the rigors of life in the plankton will eventually settle on a
suitable substrate, and undergo metamorphosis. The larval structures subsequently
wither and disappear, leaving a juvenile sea urchin (Smith et al. 2008). Drawings of
advanced larvae show the relationships of the echinus rudiment to the larva more
clearly than do microphotographs (see Fig. 7.3). The juvenile urchin forms within
the body space, the former blastocoel of the embryo while the growing larva is still
part of the plankton. A portion of the larval foregut, in association with cellular
descendants of the “small micromeres,” which are generated at the fifth cleavage
division, gives rise to a rudimentary structure (called the echinus rudiment) that
gradually forms a miniature version of the mature sea urchin. The rudiment
gradually assumes a more familiar morphology possessing the pentaradial form
characteristic of echinoderms. Early in development of the rudiment, small test
plates form and become calcified. Tube feet and spines will also become apparent.
By the time the juvenile is a few millimeters in diameter, it will have calcified test
plate elements, spines, and teeth.
The morphological details of appearance and growth of the adult endoskeleton
have been little studied. Some recent work by Yajima and Kiyomoto (2006);
(Yajima 2007) has established that cells responsible for juvenile calcified endoskeleton are not PMCs, but rather a related yet distinct embryonic lineage called
secondary mesenchyme cells. Smith et al. (2008) worked out a detailed atlas of
developmental stages of metamorphosis of Strongylocentrotus purpuratus and were
able to use specific antibodies directed against endoskeleton-specific proteins to
chart the early development of the endoskeleton. Calcified structures in the juvenile
are often found arising in close conjunction with larval spicules. Continued development of spines, test plates, and pedicellariae has been studied by electron
microscopy (Ameye et al. 1999, 2001). It is generally believed that cells closely
associated with biomineralized structures in the adult are responsible for their
deposition, a reasonable supposition, though detailed evidence is often lacking,
except for the cases of the spine and tooth, which we shall consider in due course.
Classical descriptions of the gross and microanatomy of the juvenile endoskeleton – the test plates, teeth, and spines – can be found in Hyman (1955). The
presence in the adult of a few of the characterized matrix proteins of sea urchins
has been verified by Western blotting of extracted matrix proteins (Killian and Wilt
1996) or identification of the cognate mRNAs (George et al. 1991; Livingston et al.
2006). Immuno-labelling electron microscopic localization of SM30 and SM50 in
pedicellaria and spines of the adult was done by Ameye et al. (1999). We shall
restrict subsequent discussion to more recent work on the fine structure of the
spicules, the tooth, and the spine.
7 Molecular Aspects of Biomineralization of the Echinoderm Endoskeleton
205
by Beniash et al. (1997).
7.2.4 Formation of Postembryonic Skeletal Elements
Larvae that survive the rigors of life in the plankton will eventually settle on a
suitable substrate, and undergo metamorphosis. The larval structures subsequently
wither and disappear, leaving a juvenile sea urchin (Smith et al. 2008). Drawings of
advanced larvae show the relationships of the echinus rudiment to the larva more
clearly than do microphotographs (see Fig. 7.3). The juvenile urchin forms within
the body space, the former blastocoel of the embryo while the growing larva is still
part of the plankton. A portion of the larval foregut, in association with cellular
descendants of the “small micromeres,” which are generated at the fifth cleavage
division, gives rise to a rudimentary structure (called the echinus rudiment) that
gradually forms a miniature version of the mature sea urchin. The rudiment
gradually assumes a more familiar morphology possessing the pentaradial form
characteristic of echinoderms. Early in development of the rudiment, small test
plates form and become calcified. Tube feet and spines will also become apparent.
By the time the juvenile is a few millimeters in diameter, it will have calcified test
plate elements, spines, and teeth.
The morphological details of appearance and growth of the adult endoskeleton
have been little studied. Some recent work by Yajima and Kiyomoto (2006);
(Yajima 2007) has established that cells responsible for juvenile calcified endoskeleton are not PMCs, but rather a related yet distinct embryonic lineage called
secondary mesenchyme cells. Smith et al. (2008) worked out a detailed atlas of
developmental stages of metamorphosis of Strongylocentrotus purpuratus and were
able to use specific antibodies directed against endoskeleton-specific proteins to
chart the early development of the endoskeleton. Calcified structures in the juvenile
are often found arising in close conjunction with larval spicules. Continued development of spines, test plates, and pedicellariae has been studied by electron
microscopy (Ameye et al. 1999, 2001). It is generally believed that cells closely
associated with biomineralized structures in the adult are responsible for their
deposition, a reasonable supposition, though detailed evidence is often lacking,
except for the cases of the spine and tooth, which we shall consider in due course.
Classical descriptions of the gross and microanatomy of the juvenile endoskeleton – the test plates, teeth, and spines – can be found in Hyman (1955). The
presence in the adult of a few of the characterized matrix proteins of sea urchins
has been verified by Western blotting of extracted matrix proteins (Killian and Wilt
1996) or identification of the cognate mRNAs (George et al. 1991; Livingston et al.
2006). Immuno-labelling electron microscopic localization of SM30 and SM50 in
pedicellaria and spines of the adult was done by Ameye et al. (1999). We shall
restrict subsequent discussion to more recent work on the fine structure of the
spicules, the tooth, and the spine.
7 Molecular Aspects of Biomineralization of the Echinoderm Endoskeleton
205