Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 262

Figure 5.5 The structural continuum of the cell. A wide variety of reports has demonstrated
that ECM fibers make contact with cell membrane receptors (black boxes) that are themselves
in contact with the intracellular cytoskeleton networks . Some of the cytoskeleton elements
make contact with proteins located in the nuclear envelope, which also bind to elements of
the nuclear shell (lamina). Finally, the lamina interacts with chromatin via proteins that bind
both lamins, the structural components of the lamina, and chromatin-associated proteins. A
similar continuum also exists starting from cell-cell contacts (gray boxes). Abbreviations : MF,
actin microfilaments; MT, microtubules; IF, intermediate filaments.
Source : Lelièvre (2009) .
structural elements of the nuclear matrix ( Figure 5.5 ) and are an integral part of the
nuclear skeleton ( Tolstonog et al., 2002 ).
The ECM is a noncellular network of various components secreted by cells, which
fills the intercellular spaces. The cell responds to changes in the ECM composition and
thickness with a global reorganization of chromosomes and changes in the expression
of 990 genes. The mediator and inducer of these changes is the cytoskeleton (disrup-
tion of actin filaments exposes chromatin, whereas disruption of microtubules and
intermediate filaments sequesters it) ( Lelièvre, 2009; Maniotis et al., 2005 ). Via mem-
brane receptors that have access to the ECM ( Figure 5.5 ), the cytoskeleton senses the
changes in the ECM and responds by sending signals for adaptive expression of genes.
The Evolution of the Integrated Control System in Metazoans
An Obscure Control System in Sponges
Sponges are the simplest of extant metazoans that evolved probably more than
600 Mya. Multicellular structures, believed to have been the ancestors of modern