What are the Microtubules?
Microtubules are protein-made, fine, long, hollow but rigid, un-branched tubular structures present in the cytoplasm of the cells. They are microscopic hollow tubes made of the proteins alpha and beta tubulin that are part of a cell’s cytoskeleton, a network of protein filaments that extends throughout the cell, gives the cell shape, and keeps its organelles in place and in 1963, Slautterback gave its name. Microtubules are hollow cylinders made up of repeating protein structures.
Number of Microtubules
Large numbers of microtubules are found in both plant and animal cells. These are freely distributed in the cytoplasm or remain arranged in bundles in some specialized structures such as centriole, cilia, flagella, nerve fibers, tail of sperms etc. Microtubules are of two types- (i) permanent and (ii) temporary; those present in flagella, cilia, centrioles, sperm-tail etc., are of permanent type, whereas the microtubules scattered in cytoplasm and those found in spindle fibers during cell division are temporary type. In prokaryotic cells Microtubules are not found.
Origin of Microtubules
Microtubules are formed by polymerization of a globular protein called tubulin which is present in the cytosol.
Structure of Microtubules
Microtubules are hollow cylindrical tubes having an outer diameter of approximately 25 nm. The wall of these tubes is about 5 nm in thickness and their hollow core is about 15nm in diameter. The length of the tube is changeable. The wall of each microtubule is made up of about 10-14 fine, parallel filamentous subunits called proto-filaments that are arranged longitudinally or spirally around the lumen of the microtubule. The lumen of the microtubules most likely remains filled with cytosol. The proto-filaments are composed of a special type of globular protein called tubulin.
An extraordinary property of microtubules is their configuration and breakdown caused by assembly and disassembly of the tubulin unit respectively. This assembly-disassembly process is in a active equilibrium and it is a polarized occurrence. In a microtubule, the tubulin units assemble at one end and disassemble from the other end. Colchicine is a chemical agent which prevents polymerisation and assembly of tubulin units and thus the formation of microtubules. In addition to tubulin, the microtubules also contain a number of other proteins in small quantities.
Functions of Microtubules
Following are the main functions of microtubules:
i) Microtubules form the framework or cytoskeleton of the cell and thereby give mechanical support and a shape to the cell.
ii) They help in the formation of cilia and flagella and their movement.
iii) They form spindles during cell division and help in the immigration of daughter chromosomes towards the poles.
iv) Most likely the macromolecules are transported within the cell from one part to another through these tubules.
v) Microtubules help in change of shape of the cells during cell differentiation.
vi) Microtubules present in the sensory cells may play some role in sensory transduction or formation of nerve impulses when the sensory cells are inspired.
Organels of Microtubules
Several cell organelles are derived from assemblage of microtubules. Major such organelles are centrosomes, cilia and flagella.