Essential Cell Biology 5th edition

Cancer
719
normal epithelial cell
primary tumor cell
normal liver
tissue
secondary tumors
(metastases)
cancer cells
normal
liver cells
basal
lamina
connective
tissue
(A)
blood or
lymphatic vessel
(B)
20 mm
(C)
200 µm
Cancer cells are defined by two heritable properties: they and their progeny
(1) proliferate in defiance of the normal constraints and (2) invade
and colonize territories normally reserved for other cells (Movie 20.7).
It is the combination of these socially deviant features that creates
the lethal danger. Cells that have the first property ECB5 but e20.45/20.45 not the second
proliferate excessively but remain clustered together in a single mass,
forming a tumor. Such a tumor is said to be benign, and it can usually be
removed cleanly and completely by surgery. A tumor is cancerous only if
its cells have the ability to invade surrounding tissue, in which case the
tumor is said to be malignant. Malignant tumor cells with this invasive
property often break loose from the primary tumor and enter the bloodstream
or lymphatic vessels, from where they form secondary tumors, or
metastases, at other sites in the body (Figure 20–42). The more widely
the cancer spreads, the harder it is to eradicate.
Epidemiological Studies Identify Preventable Causes of
Cancer
Prevention is always better than cure, but to prevent cancer we need to
know what causes it. Do factors in our environment or features of our
way of life trigger the disease and help it to progress? If so, what are they?
Answers to these questions come mainly from epidemiology—the statistical
analysis of human populations in search of factors that correlate with
disease incidence. This approach has provided strong evidence that the
environment plays an important part in the causation of most cases of
human cancer. The types of cancers that are common, for example, vary
from country to country, and studies of migrants show that it is usually
where people live, rather than where they were born, that governs their
cancer risk.
Although it is still hard to discover which specific factors in the environment
or lifestyle are significant, and many remain unknown, some have
been precisely identified. For example, it was noted long ago that cervical
cancer, which arises in the epithelium lining the cervix (neck) of the
uterus, was much more common in women who were sexually active
than in those who were not, suggesting a cause related to sexual activity.
We now know that most cases of cervical cancer depend on infection of
Figure 20–42 Cancers invade surrounding
tissues and often metastasize to distant
sites. (A) To give rise to a colony in a
new site—called a secondary tumor or
metastasis—the cells of a primary tumor
in an epithelium must typically cross the
protective barrier provided by the basal
lamina (yellow), migrate through connective
tissue (blue), and get into either blood or
lymphatic vessels. They then have to exit
from the bloodstream or lymph and settle,
survive, and proliferate in a new location
(not shown). (B) Secondary tumors in a
human liver, originating from a primary
tumor in the colon. (C) Higher-magnification
view of one of the secondary tumors,
stained differently to show the contrast
between the normal liver cells and the
cancer cells. (B and C, courtesy of Peter
Isaacson.)