Under the microscope, micro- 
tubules can be observed 
growing and shrinking rapidly. 
One of the most vital functions 
of microtubules is to aid in 
cell division. Just before a 
cell divides, small bodies 
called centrosomes (which are 
themselves composed of 
microtub ule-like fragments) 
migrate to the cell’s poles. An 
oval-shaped bundle made of 
microtubules forms between 
the centrosomes. Chromosomes 
attach to the bundle, which 
then helps to guide them to 
the daughter cells. In 1988, 
Kinesin 
Marc Kirschner, who was 
then at the University of 
California, San Francisco, 
and his colleagues found strong 
evidence that chromosomes 
move toward the poles as the 
microtubules slowly dissolve. 
Between cell divisions, 
microtubules act as miniature 
highways along which vesicles 
carrying such materials as 
hormones, neurotransmitters, 
and nutrients move. Using 
new techniques, such as 
video-enhanced light 
microscopy, scientists in 
several laboratories have 
observed microtubules inter- 
acting with a protein called 
kinesin that functions as a 
vesicles and organelles along 
microtubule “tracks” toward 
the cell surface. Kinesin also 
moves vesicles filled with 
neurotransmitters along the 
microtubules within nerve cell 
axons. A second, more 
recently discovered motor 
protein, called dynein, moves 
vesicles in the opposite direc- 
tion, toward the cell’s interior. 
Microtubules are also involved 
in the movement of cilia and 
flagella. These whiplike 
filaments project from certain 
cells and perform a variety of 
tasks. Large numbers of cilia 
are found on cells that line 
the respiratory tract, for 
instance, where they help to 
