Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

442 Chapter 8: Analyzing Cells, Molecules, and Systems
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Figure 8–3 Light micrographs of cells in culture. (A) Mouse fibroblasts.
(B) Chick myoblasts fusing to form multinucleate muscle cells. (C) Purified rat
retinal ganglion nerve cells. (D) Tobacco cells in liquid culture. (A, courtesy of
Daniel Zicha; B, courtesy of Rosalind Zalin; C, from A. Meyer-Franke et al.,
Neuron 15:805–819, 1995. With permission from Elsevier; D, courtesy of
Gethin Roberts.)
origin (Figure 8–3): fibroblasts continue to secrete collagen; cells derived from
embryonic skeletal muscle fuse to form muscle fibers that contract spontaneously
in the culture dish; nerve cells extend axons that are electrically excitable and
make synapses with other nerve cells; and epithelial cells form extensive sheets
with many of the properties of an intact epithelium. Because these properties are
maintained in culture, they are accessible to study in ways that are often not possible
in intact tissues.
Cell culture is not limited to animal cells. When a piece of plant tissue is cultured
in a sterile medium containing nutrients and appropriate growth regulators,
many of the cells are stimulated to proliferate indefinitely in a disorganized
manner, producing a mass of relatively undifferentiated cells called a callus. If
the nutrients and growth regulators are carefully manipulated, one can induce
the formation of a shoot and then root apical meristems within the callus, and, in
many species, regenerate a whole new plant. Similar to animal cells, callus cultures
can be mechanically dissociated into single cells, which will grow and divide
as a suspension culture (see Figure 8–3D).
Eukaryotic Cell Lines Are a Widely
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The cell cultures obtained by disrupting tissues tend to suffer from a problem—
eventually the cells die. Most vertebrate cells stop dividing after a finite number of
cell divisions in culture, a process called replicative cell senescence (discussed in
Chapter 17). Normal human fibroblasts, for example, typically divide only 25–40
times in culture before they stop. In these cells, the limited proliferation capacity
reflects a progressive shortening and uncapping of the cell’s telomeres, the repetitive
DNA sequences and associated proteins that cap the ends of each chromosome
(discussed in Chapter 5). Human somatic cells in the body have turned off
production of the enzyme, called telomerase, that normally maintains the telomeres,
which is why their telomeres shorten with each cell division. Human fibroblasts
can often be coaxed to proliferate indefinitely by providing them with the
gene that encodes the catalytic subunit of telomerase; in this case, they can be
propagated as an “immortalized” cell line.
Some human cells, however, cannot be immortalized by this trick. Although
their telomeres remain long, they still stop dividing after a limited number of divisions
because culture conditions cause excessive mitogenic stimulation, which
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