Cytoskeleton

In addition to the membrane-enclosed organelles, the cytoplasm of most cells contains a variety of protein filaments. This filamentous network is referred to as the cell's cytoskeleton (Figure 3-15), and, like the bony skeleton of the body, it is associated with processes that maintain and change cell shape and produce cell movements.

There are three classes of cytoskeletal filaments, based on their diameter and the types of protein they contain. In order of size, starting with the thinnest, they

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Physiology: The Companies, 2001 Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

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FIGURE 3-15

Cells stained to show the intermediate filament components of the cytoskeleton.

From Roy A. Quinlan, et al., Annals of the New York Academy of Sciences, Vol. 455, New York, 1985.

FIGURE 3-15

Cells stained to show the intermediate filament components of the cytoskeleton.

From Roy A. Quinlan, et al., Annals of the New York Academy of Sciences, Vol. 455, New York, 1985.

are (1) microfilaments, (2) intermediate filaments, and (3) microtubules (Figure 3-16). Microfilaments and microtubules can be assembled and disassembled rapidly, allowing a cell to alter these components of its cytoskeletal framework according to changing requirements. In contrast, intermediate filaments, once assembled, are less readily disassembled.

Microfilaments, which are composed of the contractile protein actin, make up a major portion of the cytoskeleton in all cells. Intermediate filaments are most extensively developed in regions of cells that are subject to mechanical stress (for example, in association with desmosomes).

Microtubules are hollow tubes about 25 nm in diameter, whose subunits are composed of the protein tubulin. They are the most rigid of the cytoskeletal filaments and are present in the long processes of nerve cells, where they provide the framework that maintains the processes' cylindrical shape. Microtubules radiate from a region of the cell known as the centrosome, which surrounds two small cylindrical bodies, centri-oles, composed of nine sets of fused microtubules. The centrosome is a cloud of amorphous material that regulates the formation and elongation of microtubules. During cell division the centrosome generates the microtubular spindle fibers used in chromosome separation. Microtubules and microfilaments have also been implicated in the movements of organelles within the cytoplasm. These fibrous elements form the tracks along which organelles are propelled by contractile proteins attached to the surface of the organelles.

Cilia, the hairlike motile extensions on the surfaces of some epithelial cells, have a central core of micro-tubules organized in a pattern similar to that found in

Cytoskeletal filaments Diameter (nm) Protein subunit

Microfilament 7 Actin

Intermediate filament 10 Several proteins

FIGURE 3-16

Cytoskeletal filaments associated with cell shape and motility. flf

Microtubule

Tubulin

FIGURE 3-16

Cytoskeletal filaments associated with cell shape and motility. flf

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

I. Basic Cell Functions

3. Cell Structure

Cell Structure CHAPTER THREE

Cell Structure CHAPTER THREE

the centrioles. These microtubules, in combination with a contractile protein, produce movements of the cilia. In hollow organs that are lined with ciliated epithelium, the cilia wave back and forth, propelling the luminal contents along the surface of the epithelium.

SUMMARY

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