been extensively described (see also this book, Chapter by Goldberg and Wilt).
Considerable progress has been made in elucidating the molecular basis of PMC
specification by the analysis of a gene regulatory network that operates in the large
micromere descendants (Ettensohn 2009). Upstream components include several
maternal and zygotic transcription factors; downstream are gene products that
directly control the PMCs morphogenetic behaviors, such as ingression, migration,
fusion, and deposition of the biomineralized endoskeleton. In the following, we will
focus on selected aspects of skeletogenesis concerning the external cues stimulating
PMCs to produce the right patterned skeleton. In particular, we will discuss on the
involvement of extracellular matrix (ECM) molecules and growth factors in the
process of skeletogenesis.
8.4.1 Extracellular Matrix
A great number of in vitro and in vivo studies highlight the great importance of the
ECM during morphogenesis. In fact, in addition to its function as an extracellular
space-filling scaffold, it plays a fundamental role in cell–substrate interactions,
providing both spatial and temporal information to adherent cells, thus influencing
cellular proliferation and viability, differentiation, and gene expression. Accordingly, interest has increased in recent years toward the identification, purification,
and functional studies on ECM components, along with their ligands, and other
molecules involved in cell–ECM adhesion.
The ECM of the sea urchin embryo is a very complex structure, consisting of a
number of layers with many different elements. The organization of the ECM
occurs in a highly regulated fashion during different developmental steps (for a
review see McClay et al. 1990). In addition, it must be pointed out that it is possible
to distinguish between two main ECM compartments: the apical ECM, surrounding
the embryo from fertilization of the egg, and the basal lamina, localized inside the
blastocoelic cavity, which forms during the late cleavage stage (see Fig. 8.4). So far,
no ECM molecule with an active direct role in biomineralization has been
documented in the sea urchin embryo. Nevertheless, there are a number of
evidences of indirect roles played by ECM molecules during the formation of the
skeleton. Among the ECM components present in the blastocoel, only ECM
molecules that function in supporting or directing PMCs movements have been
described until now. For example, Pamlin, a protein isolated from the basal lamina
of the Hemicentrotus pulcherrimus sea urchin embryo, has been shown to promote
PMC binding and migration in vitro (Katow 1995). In Lytechinus variegatus, PMCs
have been observed by light microscopy to directly interact with a class of ECM
fibers, localized on the basal surface of the ectoderm via their filopodia (Hodor et al.
2000). A component of such fibers is ECM3, a high-molecular-weight protein
accumulated selectively in the basal lamina adjacent to the ectoderm in all regions
except for the animal pole (Wessel and Berg 1995). ECM3 has been suggested as a
strong candidate as a PMCs substrate molecule, with a role in providing guidance
234
V. Matranga et al.
Considerable progress has been made in elucidating the molecular basis of PMC
specification by the analysis of a gene regulatory network that operates in the large
micromere descendants (Ettensohn 2009). Upstream components include several
maternal and zygotic transcription factors; downstream are gene products that
directly control the PMCs morphogenetic behaviors, such as ingression, migration,
fusion, and deposition of the biomineralized endoskeleton. In the following, we will
focus on selected aspects of skeletogenesis concerning the external cues stimulating
PMCs to produce the right patterned skeleton. In particular, we will discuss on the
involvement of extracellular matrix (ECM) molecules and growth factors in the
process of skeletogenesis.
8.4.1 Extracellular Matrix
A great number of in vitro and in vivo studies highlight the great importance of the
ECM during morphogenesis. In fact, in addition to its function as an extracellular
space-filling scaffold, it plays a fundamental role in cell–substrate interactions,
providing both spatial and temporal information to adherent cells, thus influencing
cellular proliferation and viability, differentiation, and gene expression. Accordingly, interest has increased in recent years toward the identification, purification,
and functional studies on ECM components, along with their ligands, and other
molecules involved in cell–ECM adhesion.
The ECM of the sea urchin embryo is a very complex structure, consisting of a
number of layers with many different elements. The organization of the ECM
occurs in a highly regulated fashion during different developmental steps (for a
review see McClay et al. 1990). In addition, it must be pointed out that it is possible
to distinguish between two main ECM compartments: the apical ECM, surrounding
the embryo from fertilization of the egg, and the basal lamina, localized inside the
blastocoelic cavity, which forms during the late cleavage stage (see Fig. 8.4). So far,
no ECM molecule with an active direct role in biomineralization has been
documented in the sea urchin embryo. Nevertheless, there are a number of
evidences of indirect roles played by ECM molecules during the formation of the
skeleton. Among the ECM components present in the blastocoel, only ECM
molecules that function in supporting or directing PMCs movements have been
described until now. For example, Pamlin, a protein isolated from the basal lamina
of the Hemicentrotus pulcherrimus sea urchin embryo, has been shown to promote
PMC binding and migration in vitro (Katow 1995). In Lytechinus variegatus, PMCs
have been observed by light microscopy to directly interact with a class of ECM
fibers, localized on the basal surface of the ectoderm via their filopodia (Hodor et al.
2000). A component of such fibers is ECM3, a high-molecular-weight protein
accumulated selectively in the basal lamina adjacent to the ectoderm in all regions
except for the animal pole (Wessel and Berg 1995). ECM3 has been suggested as a
strong candidate as a PMCs substrate molecule, with a role in providing guidance
234
V. Matranga et al.