Chapter 9: Cytoskeleton: Structure & Motility

The text systematically explores the three primary cytoskeletal components—microtubules, microfilaments, and intermediate filaments—beginning with the structure and assembly of microtubules. These hollow cylinders, composed of alpha-tubulin and beta-tubulin dimers, are described as polar structures that often originate from microtubule-organizing centers (MTOCs) such as centrosomes and basal bodies. Key regulatory mechanisms are explained, including the energy-dependent process of treadmilling and the phenomenon of dynamic instability, where microtubules undergo rapid cycles of growth and shrinkage driven by GTP hydrolysis. The summary highlights the impact of pharmacological agents like colchicine and vinblastine, which disrupt microtubule polymerization and are utilized in cancer chemotherapy. The discussion details the crucial role of microtubule-associated proteins (MAPs), such as tau and MAP2 for bundling, and the motor proteins kinesin and dynein that utilize ATP to facilitate anterograde and retrograde axonal transport. Significant attention is given to the ultrastructure of cilia and flagella, elucidating the characteristic 9 plus 2 arrangement and the sliding microtubule model powered by dynein arms to generate motion. The chapter then transitions to microfilaments, formed from actin, which function in maintaining cellular structures like the brush border microvilli of intestinal cells and driving contractile events such as cytoplasmic streaming and amoeboid movement. The text outlines how actin organization is modulated by binding proteins like gelsolin, filamin, and villin, and how cytochalasins can inhibit these processes. Finally, the summary covers intermediate filaments, distinguishing them by their high stability and tissue-specific expression—including keratins in epithelia, vimentin in mesenchymal cells, desmin in muscle, and neurofilaments in neurons—and explains their clinical importance as diagnostic markers for identifying the origin of tumors.