CYTOSKELETON

In most cells, the microtubules, intermediate filaments, and microfilaments
form the flexible cellular framework called the cytoskeleton
(“cell skeleton”). This latticed framework
extends throughout the cytoplasm, connecting the various organelles
and cellular components.
Microtubules are hollow, slender, cylindrical structures in
animal cells. Each microtubule is made of spiraling subunits of
globular proteins called tubulin subunits. Microtubules
function in the movement of organelles, such as secretory
vesicles, and in chromosome movement during division of the cell
nucleus. They are also part of a transport system within the cell.
For example, in nerve cells, they help move materials through the
long nerve processes. Microtubules are an important part of the cytoskeleton
in the cytoplasm, and they are involved in the overall
shape changes that cells undergo during periods of specialization.
Intermediate filaments are a chemically heterogeneous
group of protein fibers, the specific proteins of which can vary with
cell type. These filaments help to maintain cell
shape and the spatial organization of organelles, as well as promote
mechanical activities within the cytoplasm.
Microfilaments are solid strings of protein (actin) molecules
. Actin microfilaments are most highly developed in
muscle cells as myofibrils, which help muscle cells to shorten or
contract. Actin microfilaments in nonmuscle cells provide mechanical
support for various cellular structures and help form contractile
systems responsible for some cellular movements (e.g.,
amoeboid movement in some protozoa).