arranged into long, slender 
chains. Each filament is only 
6 nanometers in diameter; 
they are the thinnest of the 
cytoskeletal components. The 
role that actin filaments play 
in muscle contraction has 
been thoroughly studied over 
the past 40 years. In the 
1950’s, a British scientist, Hugh 
Huxley, proposed a model for 
muscle contraction that has 
since been shown to be cor- 
rect. According to the model, 
each muscle cell comprises 
parallel rows of actin filaments 
that alternate with rows of 
another protein, myosin. 
When stimulated by an influx 
of calcium, projecting “arms” 
of myosin “grab” the adjacent 
actin filaments and pull, 
causing the muscle cell to 
shorten. Contraction is an 
ATP-reqniring process; each 
“grab” and release by a 
myosin molecule uses up one 
molecule of ATP. In recent 
years, researchers have found 
evidence of similar actin- 
myosin interactions 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 nanome- 
ters in diameter, are the 
thickest of the cytoskeletal 
components. They were 
noticed in the mid-1950 s, but 
were seen only rarely until 
1963, when the gentle fixative 
glutaraldehyde was developed. 
Each hollow tubule is composed 
chiefly of small, spherical 
subunits of proteins called 
tubulins. Microtubules 
assemble spontaneously from 
“pools” of tubulin when 
needed and, under appropriate 
conditions, dissolve back into 
their tubulin subunits. 
(Microfilaments also form and 
break down spontaneously.) 
The microfilament 
bundles in this skin cell 
have been stained with 
modified antibodies that 
glow in response to a 
specific wavelength of 
light. 
41 
