HUMAN BIOLOGY

Genes and disease: CanCer 427

Why does a cell become cancerous?

• Cancer develops when genetic mutations remove normal
  controls over cell division.


• Cancer cells are abnormal in both their structure and the way
  they function.


• Cancer cells usually are not well differentiated (specialized).
  They can break away from a primary tumor, invade surrounding
  tissue, and metastasize to other areas of the body.

taKe-Home messaGe

metastasis The process in

which cancer cells spread

from one part of the body to

another.

A cancer cell’s structure is abnormal

Cancer is the result of a series of mutations in a cell’s genes,
as you will read in Section 22.2. One effect of these changes
is that a cancer cell’s structure is abnormal. Often, the
nucleus is much larger than usual and there is much less
cytoplasm. Cancer cells also often do not have the struc-
tural specializations of healthy cells in mature body tis-
sues. As a general rule, the less specialized cancer cells are,
the more likely they are to break away from the primary
tumor and spread the disease.
When a normal cell is transformed into a cancerous
one, more changes take place. The cytoskeleton shrinks,
becomes disorganized, or both. Proteins that are part of
the plasma membrane are lost or altered, and new, different
ones appear. These changes are passed on to the cell’s
descendants: When a transformed cell divides, its daughter
cells are cancerous cells too.

Cancer cells also do not divide normally

Contrary to popular belief, cancer cells don’t necessar-
ily divide more rapidly than normal cells do, but they do
increase in number faster. This is because the death of
normal cells usually closely balances the production of new
ones by mitosis. In a cancer, however, at any given moment
more cells are dividing than are dying. As this runaway cell
division continues, the cancer cells do not respond to crowd-
ing, as normal cells do. A normal cell stops dividing once it
comes into contact with another cell, so the arrangement of
cells in a tissue remains orderly. By contrast, a cancer cell
keeps on dividing. Therefore, cancer cells pile up in a disor-
ganized heap. This is why cancer tumors are often lumpy.
Cancer cells also do not stay well connected physically
to the cells next to them in a tissue, and they may form

extensions (pseudopodia, “false feet”)

that enable them to move about (Fig-

ure 22.3). These extensions allow

cancer cells to break away from the

parent tumor and invade other tis-

sues, including the lymphatic and circulatory systems (Fig-

ure  22.4). The spread of cancer is called metastasis. It is

what makes a cancer malignant.

Some kinds of cancer cells produce the hormone HCG,

human chorionic gonadotropin. (Recall from Chapter 17

that HCG maintains the uterus lining when a pregnancy

begins.) The presence of HCG in the blood can serve as a

red flag that a cancer exists somewhere in a person’s body.

Some cancer cells produce a chemical that stimulates

cell division, and the cells themselves have receptors for

that chemical. Cancer cells also secrete a growth factor

called angiogenin that encourages new blood vessels to

grow around the tumor. The blood vessels can “feed” the

tumor with the large supply of nutrients and oxygen it

needs to continue growing. Patients with certain types of

cancer may receive drugs that essentially starve tumors to

death by blocking the effects of angiogenin, although the

drugs can have serious side effects.

Figure 22.3 Cervical cancer cells have the threadlike “false feet”
that are a common feature of cancerous cells. Here, a cervical
cancer cell has nearly completed the process of dividing into two.
Color has been added to this scanning electron micrograph.

Figure 22.4 Animated! Cancer spreads step by step.

A Cancer cells break

away from their home

tissue.

B The metastasizing

cells become attached

to the wall of a blood

vessel or lymph vessel.

They secrete enzymes

that break down part

of the wall. Then they

enter the vessel.

C Cancer cells creep

or tumble along inside

blood vessels, then

leave the bloodstream

the same way they

got in. They start new

tumors in new tissues.

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