Nonpolar filaments that are generally found only in multicellular organisms. These intermediate sized (10 nm in diameter) filaments are thinker than their
microfilament analogs, but thinner than
microtubules, thus the name "intermediate". Intermediate filaments are structural in function, and pass through
cell junctions and intermembrane proteins to pass through several cells and integrate them into tissue (sort of like a wire mesh that provides larger structure and support).
The strength of intermediate filaments is illustrated by claws and hair, which are made up of dead
cells that are composed mainly of intermediate filaments. This stability extends to the
chemical realm as well. Intermediate filaments are not
dynamic in the way the
microtubules and
microfilaments are. Instead of constantly stacking and unstacking, it is formed by the relatively stable polymerization into a rope-like structure.
Although IFs are not as dynamic as other filaments,and there are no
motor proteins that are known to work with them, they are even more unusual in that they are composed of 6 (plus or minus depending on how you clasify) different types of subunits. The first two types (
acidic and
basic keratins) combine with each other in various ways (though always 1 acidic to 1 basic) to form the various types necessary for
epithelial cells (from that in hair, to that in the
lumen the
intestine).
The third type do not have to form with any other type (they can form "
homopolymeric" IFs), but they can. This type is found in
muscle and
glial cells, as well as
neurons. The type three
Desmin is particularly important in stabilizing the
z disk of
sarcomeres.
The fourth type is found only in the nervous system and are responsible for
axon elongation.
The fifth type is found only in lens fiber cells.
The final type is found only in the
nucleus (of all
eukaryotes) and make up what is commonly known as the "
nuclear lamin" which is a network that supports the
nuclear membrane (the role of intermediate filament associated protein (IFAP) receptor for lamin B in binding lamin B to the membranes particularly interesting).
Mutations affecting the expression of intermediate filaments can lead to horrible genetic skin
blistering diseases like
epidermolysis bullosa simplex.