ULTIMATE COMPUTING - Quantum Consciousness Studies

120 Cytoskeleton/Cytocomputer
vertices of the geodesic network and tropomyosin is localized along the actin
fibers connecting the vertices. Longer fibers attach to network vertices and extend
into filopodia, lamellipodia, and membranes. The geodesic network and vertices
act as organization centers involved in maintaining cell structure (Figure 5.26).
Figure 5.26: Geodesic actin cytomatrix gel surrounding cell nucleus, from
Lazarides and Revel (1979). Vertices of the icosahedral nuclear dome attach
filaments which extend to the cell membrane. By Paul Jablonka.
Some actin networks are transient dynamic structures which serve fleeting,
but vital functions during the cell cycle. During the final stages of cell division,
after duplicated chromosomes have been pulled apart by the mitotic spindle, a
ring of constriction encircles the equator of the mother cell perpendicular to the
spindle axis. Work by this “cleavage furrow” constricts the cytoplasm until it is
divided into two daughter cells. The constricting tension has been estimated from
observations in sand dollar and sea urchin eggs to be about 3 x 10 5 dynes per
square centimeter, a value comparable to the force generated in muscle (Lazarides
and Revel, 1979). The cleavage furrow contractile ring is a temporary structure
that exists for only about 10 minutes; it rapidly assembles and disassembles.
Although the width and thickness of the contractile ring remain constant during
constriction, the volume decreases. The ring must disassemble even as it
contracts, meaning that any sliding interaction of the actin and myosin filaments
must be followed by disaggregation of some of the filaments. This disassembly
must take place uniformly through the ring during its entire brief lifetime.
Therefore, a simple sliding filament model is insufficient to explain the cell
cleavage function of the contractile ring.
Steve Heidemann and colleagues (Joshi, Chu, Buxbaum and Heidemann,
1985) at Michigan State University have examined compressive and tensile