18.2: Overview of Cytoskeletal Filaments and Tubules

In early microscopy, eukaryotic cells looked like membrane-bound sacs of cytoplasm containing nuclei and assorted organelles. By the late nineteenth century, microscopists were describing the dynamic fibers that accompanied dramatic changes in the structure of dividing cells. No doubt you recognize this as mitosis, in which duplicated chromosomes (chromatids) materialize just as the nuclear membrane dissolves, and spindle fibers form. Over time the fibers 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 again at a fluorescence micrograph of a mitosing metaphase cell (Figure 18.1). Most of the cell, other than what is fluorescing, is not visible in the micrograph.

Figure 18.1: Drawing (left) and fluorescence micrograph (right) of a cell in metaphase of mitosis: aligned chromosomes (chromatids) at the center of the cell (blue in the micrograph) are just about to be pulled apart by microtubules of the spindle apparatus (green) extending from the poles to the center of the cell.

To get the micrograph in Figure 18.1, antibodies were made against purified microtubule, kinetochore, and chromosomal proteins (or DNA), and then linked to different fluorophores (organic molecular fluorescent tags). When the tagged antibodies were added to dividing cells in metaphase, they bound to their respective fibers. In a fluorescence microscope, the fluorophores emit different colors of visible light. 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.

As for muscle movement in whole organisms, early-to-mid-twentieth-century studies asked what the striations (stripes) seen in skeletal muscle by light microscopy might have to do with muscle contraction. The striations turned out to be composed of an isolable protein complex that investigators called 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 and in which organelles are embedded. As noted, the most abundant of these are microfilaments, microtubules, and intermediate filaments. Though myosin is less abundant, it is nonetheless present in nonmuscle cells. Microtubules account for chromosomal movements of mitosis and meiosis; and 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 the movements of the cilia and flagella, which power the movement of whole cells like paramecia, amoebas, phagocytes, and the like. Actin and myosin enable muscle contraction and thus, higher animal movement. Finally, the cytoskeleton is a dynamic structure. Its fibers not only account for the movements of cell division but also give cells their shape and mechanical strength. All the fibers can disassemble, reassemble, and rearrange, allowing cells to change shape. These changes range from the pseudopodia extended by amoeboid cells to the spindle fibers that stretch cells in mitosis and meiosis, to the constriction of a dividing cell that eventually pinches off daughter cells, and more! In this chapter we look in some detail at the roles of these tubules and filaments in cell structure and in different forms of cell motility.