THE CYTOSKELETON, 
THE CELL'S PHYSICAL PROPS 
Many cells in a multicellular organism 
must combine the seemingly contradic- 
tory traits of stability and mobility. 
With few exceptions, multicellular 
organisms begin to develop when a 
motile sperm meets an egg. Many 
cell divisions occur, and then cells 
migrate to their final positions. During 
life, individual cells divide frequently, 
and certain specialized cells move 
through the body to accomplish 
various tasks. In addition, every cell 
must have a mechanism for moving 
materials within itself. Balancing the 
need for movement is the requirement 
for cell stability. A cell must maintain 
its shape against the pressure of sur- 
rounding cells. Keeping a cell firm 
while enabling it to move are the twin 
roles played by the cytoskeleton. 
For a long time, microscopists be- 
lieved that the cytoplasm surrounding 
the cell's organelles was completely 
unstructured. But as scientists began 
to use newer and gentler fixatives to 
prepare cells for electron microscopy, 
a lacy network of fibers was revea led. 
These structures crisscross the cell like 
girders and it was hypothesized (and 
later shown experimentally) that, like 
an animal's bony skeleton, these struc- 
tures play a role in giving the cell its 
shape. For this reason, they are 
known collectively as the cytoskeleton. 
There are three main kinds of 
cytos ke I eta I f i bens — m i c rof i I a m e n ts , 
microtubules, and intermediate fila- 
ments— which are distinguishable both 
by their structure and by their protein 
composition. All three support and 
stiffen the cell. In addition to their 
structural roles, microtubules and 
microfilaments are essential for a 
variety of dynamic whole-cell activi- 
ties, including division, contraction, 
and crawling, as well as for the 
movement of vesicles within the cell. 
Microfilaments are more commonly 
called actin filaments because they 
are composed of "beads" of the 
protein actin arranged into long, 
slender chains. Each filament is only 
6 nanometers in diameter; they are 
the finest of the cytoskeletal compo- 
nents. The role that actin filaments 
play in muscle contraction has been 
thoroughly studied over the past 30 
years. In the 1 950's, a Brifish 
scientist, Hug h FHuxley, proposed a 
model for muscle contraction that has 
si.nce been shown to be correct. 
According to the model, each muscle 
cell comprises parallel rows of actin 
filaments that alternate with rows of 
another protein, myosin. When stimu- 
lated by an influx of calcium, project- 
ing "arms" of myosin "grab" the 
adjacent actin filaments and pull, 
causing the muscle cell to shorten. 
-Contraction is an ATP-requiring 
process; each "grab" and release 
uses up one molecule of ATP. In 
recent years, researchers have found 
evidence of similar actin-myosin inter- 
actions in many other kinds of cells, 
including cells that secrete hormones 
and white blood cells that move 
through the body to fight invading 
organisms. 
Microtubules, at 22 nanometers in 
diameter, are the thickest of the 
cytoskeletal components. They were 
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