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Microtubules are part of the cytoskeleton and can assemble into bundles, for example, the cortical microtubules, the spindle and the phragmoplast.
Discovery:
De Robertis and Franchi (1953) reported the presence of microtubules in cell.
Ledbetter and Porter (1963) found microtubules in the cytoplasm and termed as “Cytotubule”.
Distribution:
Fawcett and Wetbski: These are distributed throughout the cytoplasm.
Du Praw: These cytotubules/microtubules are attached from the nuclear membrane to the cell membrane.
Before the invention of electron microscope, scientists hypothesized that special types of skeletons are distributed throughout the entire cell that make compartments for other organelles. They named it as “Cytoskeleton”.
Chemical composition:
The microtubules are formed of a protein called tubulin. A tubulin subunit contains one α-tubulin molecule and one β-tubulin molecule. This αβ dimer is 80-100 Å long.
The α- and β-tubulin molecules are arranged alternately in a helical manner.
Many other proteins, called MAPs (microtubule associated proteins), form some 5 to 10 percent of the proteins of microtubules. These proteins promote tubulin polymerization.
Fine Structure:
The microtubules are hollow, unbranched cylinders, generally about 200 to 270 Å thick and several micrometers long.
They may occur singly or in bundles, and radiate from the centriole to the periphery of the cell.
The microtubule is composed of 13 parallel protofilaments that run its entire length and enclose a central lumen about 150 Å wide.
Each proto-filament is made up of a row of globular subunits that have a diameter of about 40 to 50 Å. There may be cross bridges between adjacent microtubules.
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Functions of Microtubules :
- Form and support– The microtubules form a part of cytoskeleton which (a) maintains the shape of the cell and (b) provides mechanical support to the cell.
- Movement- The microtubules form the motile elements of cilia and flagella.
- Components of centriole and basal bodies- The microtubules are components of centriole and basal bodies. The centriole give rise to the mitotic spindle and the basal bodies produce cilia and flagella.
- Formation of mitotic spindle- The microtubules forms the spindle and astral rays in cell division.
- Chromosome movement– The chromosome fibers of spindle bring about movement of the chromosomes to the opposite poles of the cell in the anaphase.
- Cell differentiation– The microtubules play a role in cell differentiation and determination of polarity.
- Intracellular transport- Vesicles and protein molecules in the cell move along the “tracks” of microtubules.
