interaction, such as secreted proteases. In conclusion, although for some of those
proteins the biological functions have been proved, the future challenge will be to
characterize and demonstrate the specific role of each protein in the growth and
patterning of the skeleton.
8.3 Cells Involved in Adult Echinoderms Biomineralization
Teeth, spines, and the test of adult echinoderms are all formed by a terminally
differentiated type of cells of mesodermal origin, called sclerocytes, specializing in
the deposition of skeletal parts. Sclerocytes are the only cell population to contact
the stereom and occupy the mesodermal stroma tissue of an intact mineral structure.
They produce thin cell processes that closely surround the trabeculae, forming the
so-called skeletal cytoplasmic sheath (Heatfield and Travis 1975). The thin processes show no organelles, while the cell body includes a well-developed and
dilated Golgi complex, with associated vesicles and other organelles (Stricker
1985; M€ arkel et al. 1986; Dubois and Ameye 2001). Calcifying vacuoles are
formed within the cytoplasmic layers, which are separated from the calcite by a
vascular space. All developing ossicles appear to be surrounded by a syncytial
network of sclerocytes, though it is reported that the ossicle rudiment appears either
in a single cell (Crinoids, Echinoids, and Ophiuroids) or in a syncytium
(Holothuroids) (Dubois and Chen 1989). The mineral skeleton is produced intracellularly or intrasyncytially (M€ arkel and R€ oser 1985). During the calcification
process, the vacuoles increase in size and ramify, producing the characteristic
projection of the primary plates, and new sclerocytes are incorporated in the
pseudopodial syncytium (Kniprath 1974; M€ arkel et al. 1986). This syncytium
may either envelop preexisting calcite surface on which the biomineral grows, or
form independently from the preexisting stereom (Heatfield and Travis 1975).
8.3.1 Biomineral Formation and Regenerative Events
Echinoderms show a remarkable self-repairing ability as adults and a latent potential for regeneration of large parts of their bodies. This potential is common to all
classes of the phylum as an adaptive mechanism for survival and dispersion. Lines
of cells preserve the ability to differentiate into somatic and germ tissues, but little
is known concerning the nature of cells and the molecular pathways involved in the
different phases of echinoderm regeneration such as wound healing, growth, morphogenesis, and cell differentiation. Regeneration is in fact a characteristic type of
developmental process that can involve cell turnover and tissue repair, reconstruction of external and internal organs, and regrowth of new complete adults from
detached body fragments (Candia Carnevali et al. 2009). It has been reported that
the dermis of regenerating spines is characterized by active sclerocytes, the
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V. Matranga et al.
proteins the biological functions have been proved, the future challenge will be to
characterize and demonstrate the specific role of each protein in the growth and
patterning of the skeleton.
8.3 Cells Involved in Adult Echinoderms Biomineralization
Teeth, spines, and the test of adult echinoderms are all formed by a terminally
differentiated type of cells of mesodermal origin, called sclerocytes, specializing in
the deposition of skeletal parts. Sclerocytes are the only cell population to contact
the stereom and occupy the mesodermal stroma tissue of an intact mineral structure.
They produce thin cell processes that closely surround the trabeculae, forming the
so-called skeletal cytoplasmic sheath (Heatfield and Travis 1975). The thin processes show no organelles, while the cell body includes a well-developed and
dilated Golgi complex, with associated vesicles and other organelles (Stricker
1985; M€ arkel et al. 1986; Dubois and Ameye 2001). Calcifying vacuoles are
formed within the cytoplasmic layers, which are separated from the calcite by a
vascular space. All developing ossicles appear to be surrounded by a syncytial
network of sclerocytes, though it is reported that the ossicle rudiment appears either
in a single cell (Crinoids, Echinoids, and Ophiuroids) or in a syncytium
(Holothuroids) (Dubois and Chen 1989). The mineral skeleton is produced intracellularly or intrasyncytially (M€ arkel and R€ oser 1985). During the calcification
process, the vacuoles increase in size and ramify, producing the characteristic
projection of the primary plates, and new sclerocytes are incorporated in the
pseudopodial syncytium (Kniprath 1974; M€ arkel et al. 1986). This syncytium
may either envelop preexisting calcite surface on which the biomineral grows, or
form independently from the preexisting stereom (Heatfield and Travis 1975).
8.3.1 Biomineral Formation and Regenerative Events
Echinoderms show a remarkable self-repairing ability as adults and a latent potential for regeneration of large parts of their bodies. This potential is common to all
classes of the phylum as an adaptive mechanism for survival and dispersion. Lines
of cells preserve the ability to differentiate into somatic and germ tissues, but little
is known concerning the nature of cells and the molecular pathways involved in the
different phases of echinoderm regeneration such as wound healing, growth, morphogenesis, and cell differentiation. Regeneration is in fact a characteristic type of
developmental process that can involve cell turnover and tissue repair, reconstruction of external and internal organs, and regrowth of new complete adults from
detached body fragments (Candia Carnevali et al. 2009). It has been reported that
the dermis of regenerating spines is characterized by active sclerocytes, the
230
V. Matranga et al.