Intermediate filaments (IF) are the most flexible
structures of the cytoskeletal system. They are intermediate in diameter
compared to the larger microtubules and smaller actin filaments.
There are at least 65 different human IFs and they range in size
from about 9-11 nm. They are found only in eukaryotic cells, although
IF like proteins are present in prokaryotes.
Intermediate filaments give integrity to the nuclear envelope and
cell surface membrane. One type of IF, keratin, makes up hair, nails,
horns, and reptilian scales. Another, vimentin, anchors cell organelles
and is found attached to the nucleus, endoplasmic reticulum, and
This picture is an cryo electron micrograph
of vimentin intermediate filaments. These samples have been embedded
show the flexible regions where the IFs have unraveled from a
The basic building block of vimentin like
all other IFs is a monomer with a central alpha helical region
flanked on one end by a non helical, amino, head domain and on
the other side, by a carboxy tail. These monomers twist together
to form dimers, two dimers form a tetramers, and eight tetramers
make up the ULF, unit length filament. Unit length filaments
assemble into full length IFs.