THE CYTOSKELETON, THE CELL'S PHYSICAL PROPS 
40 
I any cells in a 
multicellular 
organism 
must combine 
the seemingly 
contradictory traits of stabil- 
ity 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 vari- 
ous tasks. In addition, every 
cell must have a mechanism 
for moving materials within 
itself. Balancing the need for 
movement is the cell’s need to 
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 
believed 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 
revealed. These structures 
crisscross the cell like girders 
and it was hypothesized (and 
later shown experimentally) 
that, like an animal’s bony 
skeleton, these structures 
play a role in giving the cell its 
shape and support. For this 
reason, they are known col- 
lectively as the cytoskeleton. 
There are three main kinds of 
cytoskeletal fibers- — microfila- 
ments, microtubules, and 
intermediate filaments — 
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 activities, 
including division, contraction, 
and crawling, as well as for 
the movement of vesicles and 
chromosomes within the cell. 
Microfilaments are more 
commonly called actin fila- 
ments because they are 
composed of “beads” 
of the protein actin 
|T stained with modified anti- 
bodies that attack to the ceils 
■scaffolding, ' called the cytoskeleton, 
and that alow in response to a specific wave- 
length of light. 
