oviduct produces a sort of current 
that draws the egg into the uterus, 
while the rapidly thrashing flagella of 
sperm help them to "swim" toward 
the egg. 
The inherent ability of microtubules 
and microfilaments to assemble and 
disassemble rapidly allows for the 
construction and destruction of these 
cytoskeletal components to suit the 
needs of a moving cell. In contrast, 
intermediate filaments are the most 
stable of the cytoskeletal fibers. At 
8 to 1 0 nanometers in diameter, they 
are intermediate in size between 
microfilaments and microtubules. 
Intermediate filaments appear in 
many types of cells, and their precise 
protein composition depends upon 
their location. They are prominent 
in parts of the cell that are under 
mechanical stress, such as the long 
axons of nerve cells and the surface 
of skin cells. 
Some researchers have reported 
that a number of diseases, including 
Alzheimer's disease and Parkinson's 
disease, are associated with changes 
in the intermediate filament arrays in 
cells. However, while intermediate 
filaments obviously have important 
functions in cell physiology, very little 
is now known about the cause or 
effect of their alteration during 
disease. 
Nevertheless, an understanding 
of intermediate filaments is proving 
valuable in tumor diagnosis. If can- 
cerous cells arise in one organ and 
then migrate to another, a clinician 
can study the intermediate filament 
composition of the tumor cells to 
Some microorganisms are equipped 
with a flagellum (composed of 
microtubules), which thrashes to 
propel the animal. 
44 
