Essential Cell Biology 5th edition

722 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
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(A)
(B)
Figure 20–44 Cancer cells often have highly abnormal chromosomes, reflecting genetic instability. Shown
here are karyotypes displaying the chromosomes of (A) a normal human cell and (B) a breast cancer cell. The
chromosomes are “painted” with a combination of fluorescent stains that give each chromosome a different
color. The breast cancer karyotype shows multiple translocations, including two instances of a translocation
ECB5 n20.102/20.47
of material from chromosome 6 (red) to chromosome 4 (light blue), one of which also includes a piece of
chromosome 1 (yellow). Whereas the normal cell contains 46 chromosomes, the breast cancer cell has 51; several
of its chromosomes are missing, and it has an extra copy of a handful of chromosomes—along with 6 copies of
chromosome 19. Such abnormalities in chromosome number can cause chromosome-segregation errors when the
cell divides, so that the degree of genetic disruption goes from bad to worse over time (see Table 20–1). (Courtesy
of Mira Grigorova and Paul Edwards.)
a mutation gives one
cell an advantage
normal
epithelial cells
growing on
basal lamina
precursor cell in the epithelial lining of the gut, for example, must undergo
changes that permit it to carry on dividing when it would normally stop
(see Figure 20–35). That cell and its progeny must also be able to avoid
cell death, displace their normal neighbors, and attract a blood supply
to nourish continued tumor growth (Movie 20.9). For the tumor cells to
then become invasive, they must be able to detach from the epithelial
sheet and digest their way through the basal lamina into the underlying
connective tissue. To spread to other organs and form metastases, they
must be able to get in, and then out, of blood or lymph vessels and settle,
survive, and proliferate in new sites (see Figure 20–42).
a second mutation increases the advantage
a third mutation
increases the
advantage further
and makes the
cell invasive
CELL SURVIVAL AND
PROLIFERATION
CELL SURVIVAL AND
PROLIFERATION
Different cancers display different combinations of properties.
Nevertheless, we can draw up a general list of characteristics that distinguish
cancer cells from normal cells.
1. Cancer cells have a reduced dependence on signals from other cells
for their survival, growth, and division. Often, this is because they
contain mutations in components of the cell signaling pathways that
normally respond to such stimuli. An activating mutation in a Ras
gene (discussed in Chapter 16), for instance, can cause an intracellular
signal for proliferation even in the absence of the extracellular cue
that would normally be needed to turn Ras on—like a faulty doorbell
that rings even when nobody is pressing the button.
DANGEROUS
CELL SURVIVAL,
PROLIFERATION,
AND INVASION
Figure 20–45 Tumors evolve by repeated rounds of mutation,
proliferation, and natural selection. The final outcome is a fully
malignant tumor. At each step, a single cell undergoes a mutation
that enhances its ability to proliferate, or survive, or both, so that its
progeny become a dominant clone in the tumor. Proliferation of this
clone then hastens occurrence of the next step of tumor progression
by increasing the size of the cell population at risk of undergoing an
additional mutation. Some cancers contain multiple malignant clones,
each with its own collection of mutations, in addition to a common set
of mutations that reflect the tumor’s origin from a founding mutant cell
(not shown).