Cytoskeleton

1.

Microtubules

Microtubules can be observed within some subcellular structures including, cilia, flagella, centrioles and the mitotic spindle, as well as forming the network/structure of the cell's cytoskeleton - as in the diagram above.

Microtubules are long hollow rod-like structures, approx 25nm in diameter and as long as 25μm - though some sources indicate up to a few mm in length. They consist of longitudinally arranged filaments called protofilaments; about 13 such filaments being arranged to form tubes i.e. hollow open-ended cylinders. The protofilaments consist of tubulin molecules known as alpha-tubulins (α-tubulins) and beta-tubulins (β-tubulins), the difference between these two forms of tubulin being their amino acid sequences. Tubulin is a protein.

Microtubules can increase or decrease in length and can build and break cross-bridges. They form a major part of the structure of the cytoskeleton within cells and help enable the structure of the cytoplasm to change by continuous modification.

See also the structure of microtubules and the functions of microtubules.

2.

Intermediate Filaments

Intermediate filaments are defined largely in terms of their size i.e. having a diameter 8-11nm, often stated as approx. 10 nm. They are generally more stable than either microtubules or microfilaments as they do not readily dissociate into monomers.

Most intermediate filaments are found in the cell cytosol between the nuclear envelope (nuclear membrane) and the plasma membrane (cell membrane).

Function of intermediate filaments:
Intermediate filaments participate in the processes by which cells change shape. They are less common in plant cells (in which cell shape is determined by the plant cell wall) than in animal cells (which have cell membranes, but not cell walls).

Examples of intermediate filaments include:

• Keratin fibres in epithelial cells
• Desmin filaments in muscle cells
• Glial filaments and neural filaments in the cells in the nervous system (see nervous tissue)
• Vimentin filaments (in many types of cells)

The above includes some of the 6 categories of intermediate filaments (6 types of intermediate filaments), specifically:

1. Acidic Keratins - part of network from nucleus to cell membrane (epithelial cells)
2. Basic Keratins - part of network from nucleus to cell membrane (epithelial cells
3. Desmin, vimentin - provide structure & mechanical support
4. Neurofilaments, synemin, syncoilin - provide structural support & integrity to cells
5. Nuclear lamina - in cell nuclei
6. Nestin - mainly in nerve cells

3.

Microfilaments

The main component of microfilaments is the protein actin.
There are different forms of actin (α, β and γ actins), of which α-actins play an important role in the thin filaments of muscle tissue* and β-actins and γ-actins are present within the microfilaments of cytoskeleton of many types of cells.

*Actin molecules are particularly known for the role they play in how muscles work, i.e. actin molecules are the points of attachment (on the thin filaments) for the myosin heads (which are part of the thick filaments). See muscle cells for further detail.

Amino acid sequences distinguish non-muscle actins differ from muscle actin. The different forms of actin can be separated using special electrophoretic techniques called "electrofocussing".

Functions of microfilaments:

• Movement of organelles inside cells e.g. chloroplasts move in response to light.
• Help with movement of whole cells, i.e. cell motility.
• Actin-binding proteins help with the attachment of microfilaments within the cytoskeleton to the membrane structures such as the cell membrane.