Functions of Filaments and Microtubules

1. Maintenance of cell shape. Since microtubules are fairly rigid they form a supporting framework or cytoskeleton and give shape to the cell. They thus maintain the shape of long processes like the axons of nerve cells, the axopodia of some Protozoa, cilia and flagella. The biconcave disc shape of erythrocytes is also maintained by microtubules.

2. Cell membrane movements. Microfilaments act as cell muscles and are involved in cell membrane movements taking place during projection of pseudopodia, endocytosis, projection and retraction of microvilli and formation of ruffles on the cell surface.

3. Movement of cilia and flagella. Microtubules making up the axial support of cilia and flagella are involved in their movement. There are two main theories to explain ciliary and flagellar movement, localized contraction and a sliding mechanism. The latter theory has more support.

4. Chromosomal movements in cell division. The spindle fibers of mitotic and meiotic spindles consist of bundles of microtubules. According to the currently favoured assembly-disassebmly hypothesis, shortening of kinetochore microtubules by disassembly and/ or elongation of microtubules of continuous fibers by assembly provides the force for chromosome movement during anaphase.

5. Transport of cellular material. Neurofilaments and microtubules in the axons of nerve cells are believed to function in the transport of proteins and other substances synthesized in the cell body down the axon. In the protozoan Actinospherium microtubules support the long, thin pseudopodia within which cytoplasmic particles show up and down streaming movements. In melanocytes movements of pigment granules takes place in the channels formed in the cytoplasm by the microtubules. Microtubules are also involved in the release of insulin from the β-cells of the pancreas.

6. Morphogenesis. Cells change their shape during cell differentiation. Elongation of the cells in the lens of the eye and the nucleus of the spermatid during sperm formation are examples of change of shape brought about by microtubules.

7. Sensory transduction. Microtubules have been found in many sensory cells in association with nerve fibers. It is possible that they act as transducers, converting stimuli into nerve impulses.