26.8 : Assembly of Complex Microtubule Structures

Complex microtubule structures are present in resting cells and in dividing cells. In resting cells, they are responsible for maintaining the cellular architecture, tracks for intracellular transport, positioning of organelles, assembly of cilia and flagella. They mediate the bipolar spindle assembly for chromosomal segregation and positioning of the cell division plate in dividing cells. The formation of microtubule complex structures depends on the cell type, cell stage, and cell function. Microtubule-associated proteins (MAPs), motor proteins, and post-translational modifications stabilize these structures.

In differentiated resting cells like neurons, the microtubule complex and other cytoskeletal filaments such as microfilaments and intermediate filaments are responsible for building and maintaining the cellular architecture. Together, microtubules and actin filaments control the direction of intricate cytoskeletal network formation for relaying the electrical signals across neurons. During the growth of axonal cones, microtubules regulate their initiation, migration, polarization, and differentiation. The neuronal cytoskeleton has heterogeneous microtubules that differ in polarity, orientation, stability, and associated proteins.

In mature epithelial cells, microtubule complexes regulate the epithelial junctions. In these cells, the nucleation of microtubules from microtubule originating centers (MTOCs) is sparse. The microtubules here gradually lose their plus-end dynamic activity where both assembly and disassembly of tubulin subunits are stabilized. The loss of dynamic activity allows mature microtubules to be arranged within the cell, with the plus-end directed towards the apical region. At the same time, the minus-end remains towards the basal side. This polarises the epithelial cell, helping form the junctions towards the apical end. Junctional proteins, such as cingulin and paracingulin, help organize microtubules within the epithelial cells.