Figure 3.45 The cytoskeleton is a network of microfilaments, intermediate filaments, and microtubules found
throughout the cytoplasm of a eukaryotic cell. In these fluorescently labeled animal cells, the microtubules are green,
the actin microfilaments are red, the nucleus is blue, and keratin (a type of intermediate filament) is yellow.
Microfilaments are composed of two intertwined strands of actin, each composed ofactinmonomers forming
filamentous cables 6 nm in diameter[25](Figure 3.46). The actin filaments work together with motor proteins, like
myosin, to effect muscle contraction in animals or the amoeboid movement of some eukaryotic microbes. In ameboid
organisms, actin can be found in two forms: a stiffer, polymerized, gel form and a more fluid, unpolymerized soluble
form. Actin in the gel form creates stability in the ectoplasm, the gel-like area of cytoplasm just inside the plasma
membrane of ameboid protozoans.
Temporary extensions of the cytoplasmic membrane calledpseudopodia(meaning “false feet”) are produced through
the forward flow of soluble actin filaments into the pseudopodia, followed by the gel-sol cycling of the actin
filaments, resulting in cell motility. Once the cytoplasm extends outward, forming a pseudopodium, the remaining
cytoplasm flows up to join the leading edge, thereby creating forward locomotion. Beyond amoeboid movement,
microfilaments are also involved in a variety of other processes in eukaryotic cells, including cytoplasmic streaming
(the movement or circulation of cytoplasm within the cell), cleavage furrow formation during cell division, and
muscle movement in animals (Figure 3.46). These functions are the result of the dynamic nature of microfilaments,
which can polymerize and depolymerize relatively easily in response to cellular signals, and their interactions with
molecular motors in different types of eukaryotic cells.
- Fuchs E, Cleveland DW. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.”Science279 no. 5350
(1998):514–519.
122 Chapter 3 | The Cell
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