Intermediate filaments are only found in multicellular organisms. Compare with the other components of the cytoskeleton, there are at least 50 different kinds of intermediate filaments, usually particular to a few cell types.
They generally fall into six molecular classes, based on amino acid sequence. They share the same general structure, being made up by fibrous proteins of the keratin family, similar to the protein that makes up hair and fingernails. In cells, these proteins are organized into tough, rope-like gathers 8 to 12 nm in diameter.
Intermediate filaments have two main structural functions: They stabilize cell structure, and they can counteract the tension. In some cells, intermediate filaments radiate from the nuclear envelope and may keep the locations of the nucleus and other organelles in the cell. The lamina of the nuclear lamina are intermediate filaments. Other kinds of intermediate filaments can help hold a complicated organs of microfilaments in place in muscle cells. And other kinds also stabilize and help maintain rigidity in surface tissues by connecting "spot welds" called desmosomes between adjacent cells.(Purves 80-81)
The intermediate filaments are grouped into six types based on similarities in amino acid sequence and protein structure.
- Type I: acidic cytokeratins.
- Type II: basic cytokeratins.
- Type III: vimentin, desmin, GFAP and peripherin in muscle cells and fibroblasts.
- Type IV: neurofilaments, α-internexina in neurons, especially in axons, synemin and syncoilin.
- Type V: lamins, fibrous proteins having structural function in the cell nucleus, that lie below the nuclear envelope.
- Type VI: nestin, present in stem cells.