18.2: Cytoskeletal Components
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Most eukaryotic cells look like a membrane-bound sac of cytoplasm containing a nucleus and assorted organelles in a light microscope. In the late 19th century, microscopists described a dramatic structural re-organization of dividing cells. In mitosis, duplicated chromosomes (i.e., chromatids) condense in the nucleus just as the nuclear membrane dissolves. Spindle fibers emerge and then seem to pull the chromatids apart to opposite poles of the cell. Spindle fibers turn out to be bundles of microtubules, each of which is a polymer of tubulin proteins. Let’s look below at that fluorescence micrograph of a mitosing metaphase cell again; most of the cell other than what is fluorescing is not visible in the micrograph.
To get this image, antibodies were made against purified microtubule, kinetochore and chromosomal proteins (or DNA), and then linked to different fluorophores (organic molecular fluorescent tags). When the fluorophores were added to dividing cells in metaphase, they bound to their respective fibers. Upon UV light irradiation, the fluorophores emit different colors of visible light, visible in a fluorescence microscope. Microtubules are green, metaphase chromosomes are blue and kinetochores are red in the micrograph.
Both mitosis and meiosis are very visible examples of movements within cells, both already described by the late 19th century. As for movement in whole organisms, mid20th century studies focused on what the striations (or stripes) seen in skeletal muscle in the light microscope might have to do with muscle contraction. The striations turned out to be composed of a protein complex originally named actomyosin (acto for active; myosin for muscle). Electron microscopy later revealed that actomyosin (or actinomyosin) is composed of thin filaments (actin) and thick filaments (myosin) that slide past one another during muscle contraction.
Electron microscopy also hinted at a more complex cytoplasmic structure of cells in general. The cytoskeleton consists of fine rods and tubes in more or less organized states that permeate the cell. The most abundant of these cytoskeletal components are microfilaments, microtubules and intermediate filaments. But, even myosin is present in non-muscle cells, albeit at relatively low concentrations. Microtubules account for chromosome movements of mitosis and meiosis, while together with microfilaments (i.e., actin), they enable organelle movement inside cells (you may have seen cytoplasmic streaming of Elodea chloroplasts in a biology lab exercise). Microtubules also underlie cilia- and flagella-based motility of whole cells such as paramecium, amoeba, phagocytes, etc., while actin microfilaments and myosin enable muscle and thus higher animal movement! Finally, the cytoskeleton is a dynamic structure. Its fibers not only account for the movements of cell division, but they also give cells mechanical strength and unique shapes. All of the fibers can disassemble, reassemble and rearrange, allowing cells to change shape, for example, creating pseudopods in amoeboid cells and spindle fibers of mitosis and meiosis. In this chapter we look at the molecular basis of cell structure and different forms of cell motility.