These are elongated, unbranched, cylindrical tubes approximately 200-250A in diameter and several microns in length. Each tubule consists of a light central core of about 150Å in di­ameter and a wall of microtubules con­sisting of 11-13 longitudinal strands. Each strand is made of poly­mers of tubulin, a protein. Micro­tubules occur singly or in bundles in the cytoplasm. They form the skeleton of cilia and flagella of motile cells, where they are arranged in a specific and characteristic manner. Two central microtubules are surrounded by nine pairs a microtubules.

The spindle of dividing cells are also formed of these microtubules. They maintain the form of the cell, its mobility and movement of chromosomes during cell division. Microtubules are dynamic structures undergoing constant addition or break­ing of tubulin at both ends, Both the end points of microtubule possess po­larity.

Alkaloids, like colchicine and vin­blastine inhibit microtubule assembly by binding with the tubulin. So, microtubules are important in move­ment of chromosomes during mitosis. They are believed to maintain the organi­sation of the Golgi complex. Associated with microfilaments, they form the mi­cro-trabecular lattice or matrix.