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, or depolymerize, back into 
their tubulin subunits. (Microfilaments 
also form and break down spontane- 
ously.) Under the microscope, microtu- 
bules can be observed growing and 
shrinking rapidly. Because microtubules 
perform so many important functions 
within the cell, scientists are eager to 
learn the details of their dynamics. 
One of the most vital functions of 
microtubules is to aid in cell division. 
Just before a cell divides, small bodies 
called centrioles (which are them- 
selves composed of microtubules) 
migrate to the cell's poles. A spindle 
made of microtubules forms between 
the centrioles. Chromosomes attach 
to the spindle, which then helps to 
guide them to the daughter cells. In 
1988, M arc Kirschner and his col- 
leagues at the University of Califor- 
nia, San Francisco, found strong 
evidence that chromosomes move 
toward the poles as the microtubules 
slowly dissolve. This indicates that, 
contrary to what many researchers 
had assumed, microtubule depolym- 
erization and subsequent chromo- 
some movement apparently do not 
require ATP. 
The spindle is easily disrupted by 
a number of chemicals. Colchicine, 
an extract of meadow saffron used 
The microfilament bundles 
in this skin cell have been stained 
with modified antibodies that glow 
under ultraviolet light. 
42 
