Chapter 13: Cytoskeletal Systems & Structural Support

Beyond acting as a mechanical scaffold, this system is critical for motility, intracellular trafficking, signal transduction, and the physical division of cells. Eukaryotic cells utilize three primary components: microtubules, microfilaments, and intermediate filaments, each defined by unique protein subunits and biochemical properties. Microtubules are hollow cylinders made of alpha-tubulin and beta-tubulin heterodimers that exhibit dynamic instability, a process where they switch between phases of growth and rapid shrinkage depending on the presence of a protective GTP cap. These structures typically originate from microtubule-organizing centers like the centrosome and are regulated by specialized proteins such as MAPs, which stabilize bundles, and catastrophins, which promote disassembly. Microfilaments, composed of polymerized actin, facilitate contractile movements and the formation of surface protrusions like lamellipodia and filopodia. Their organization is tightly controlled by actin-binding proteins and molecular switches known as Rho GTPases, which respond to extracellular signals to remodel the cell's periphery. Some pathogens, such as the bacterium Listeria, even exploit this actin machinery to propel themselves through host tissues. Intermediate filaments, the most stable and diverse group, consist of fibrous proteins like keratin and vimentin. They are categorized into several classes based on tissue specificity and are primarily responsible for resisting mechanical stress. Unlike the other two systems, intermediate filaments lack inherent polarity. Ultimately, these three networks are not isolated; they are mechanically integrated by linker proteins like plectin and spectraplakins to form a resilient and functional cellular framework that can be studied using advanced visualization techniques like fluorescence and electron microscopy.