Cytoskeleton

Definition
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Network of each cell consisting of thin, thread-like protein structures (filaments). The cytoskeleton is responsible for the mechanical stability of a cell as well as for active movements of a cell. It also performs sensory functions that are important for signal transmission between cells.

Classification
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A distinction is made between 3 classes of cytoskeletal filaments, each of which is formed by different proteins or protein classes that have specific accompanying proteins and each of which participates in the tasks of the cytoskeleton in a different way:
- Actin filaments,
- intermediate filaments,
- microtubules.
Actin filaments (also called microfilaments) are fibers with a diameter of 7 nm. They are arranged like a net in the cell, stabilize its outer shape and fix membrane-bound proteins in place. The motor proteins of actin form the protein class of myosins. The movement of the muscles is mainly based on interactions between actin and myosin.
Intermediate filaments: These are a series of protein filaments with a diameter of about 10 nm (8-11 nm). They are significantly more stable than microtubules and actin filaments and primarily absorb mechanical tensile forces. Thus, they mainly serve to mechanically stabilize the cells and form their supporting framework. 6 intermediate filaments are distinguished:
- acidic keratins: occur in epithelia
- basic keratins: occur in epithelia
- Desmine:
  - GFAP (glial fibrillar acidic protein): Occurrence in glial cells, astrocytes
  - Desmin (muscle cells)
  - Vimentin (cells of mesenchymal origin)
  - Peripherin (found in peripheral neurons)
- alpha-internexin (occurring in the CNS during development)
- Laminins:
  - Laminin A, lamin B, lamin C (occurring in the nucleus)
- Neurofilaments:
  - NF-L, beta-internexin, NF-M, NF-A (occurring in neurons)
  - Nestin (neuroepithelial stem cells)
Microtubules: Hollow cylinders with a diameter of 25 nm. They are composed of the protein tubulin. Intracellularly, with their motor proteins dynein and kinesin, they are responsible for longer transport processes and the movement and attachment of organelles in the cytosol.