104
UNIT ONE
expression not always detectable by observing cancer cells
under a microscope. Many cancers are treatable with sur-
gery, radiation, chemicals (chemotherapy), or immune sys-
tem substances used as drugs. A newer approach to treating
cancer is to develop molecules that bind to receptors unique
to, or unusually abundant on, cancer cells, blocking the cells
from receiving signals to divide.
Two major types of genes cause cancer.
Oncogenes
are
abnormal variants of genes that normally control the cell cycle,
but are overexpressed, increasing cell division rate.
Tumor
suppressor genes
normally hold mitosis in check. When
tumor suppressor genes are removed or otherwise inactivated,
this lifts control of the cell cycle, and uncontrolled cell division
leading to cancer results
(f g. 3.39)
. Cancer cells are said to be
“immortal.” Environmental factors, such as exposure to toxic
cycle. Another internal infl
uence is cell size, speciF cally the
ratio between the surface area the cell membrane provides
and the cell volume. The larger the cell, the more nutrients
it requires to maintain the activities of life. However, a cell’s
surface area limits the number of nutrient molecules that can
enter. Volume increases faster than does surface area, so a
cell can grow too large to efF
ciently obtain nutrients. Cell
division solves this growth problem. The resulting daughter
cells are smaller than the original cell and thus have a more
favorable surface area-to-volume relationship. They require
less energy and fewer nutrients, and diffusion is faster.
External controls of cell division include hormones and
growth factors. Hormones are biochemicals manufactured in
a gland and transported in the bloodstream to a site where
they exert an effect. Hormones signal mitosis in the lining
of a woman’s uterus each month, building up the tissue to
nurture a possible pregnancy. Similarly, a pregnant woman’s
hormones stimulate mitosis in her breasts when their func-
tion as milk-producing glands will soon be required.
Growth factors are like hormones in function but act
closer to their sites of synthesis. Epidermal growth factor, for
example, stimulates growth of new skin beneath the scab on a
skinned knee. Salivary glands also produce this growth factor.
This is why an animal’s licking a wound may speed healing.
Many people with cancer benef
t From drugs that a±
ect growth Fac-
tors. Granulocyte colony stimulating Factor (G-CS², sold under several
brand names) is given as a drug to boost white blood cell counts,
which plummet during chemotherapy. In contrast, anti-angiogenesis
drugs work oppositely on vascular endothelial growth Factor (VEG²),
cutting o±
a tumor’s blood supply.
Space availability is another external factor that infl u-
ences the timing and rate of cell division. Healthy cells do
not divide if they are surrounded by other cells, a phenom-
enon called contact (density dependent) inhibition.
Control of cell division is absolutely crucial to health.
With too infrequent mitoses, an embryo could not develop,
a child could not grow, and wounds would not heal. Too
frequent mitoses or those that continue unabated produce an
abnormal growth, or neoplasm, which may form a disorga-
nized mass called a
tumor.
Tumors are of two types. A
benign
tumor remains in
place like a lump, eventually interfering with the function of
healthy tissue. A
malignant,
or cancerous, tumor looks dif-
ferent—it is invasive, extending into surrounding tissue. A
growing malignant tumor may roughly resemble a crab with
outreaching claws. The word “cancer” comes from the Latin
for "the crab." Cancer cells, if not stopped, eventually reach
the circulation and spread, or metastasize, to other sites.
Table 3.5
lists characteristics of cancer cells, and
f gure 3.38
illustrates how cancer cells inF
ltrate healthy tissue.
Cancer is a collection of disorders distinguished by their
site of origin, the affected cell type, and differences in gene
TABLE
3.5
|
Characteristics of Cancer Cells
Loss oF cell cycle control
Heritability (a cancer cell divides to Form more cancer cells)
Transplantability (a cancer cell implanted into another individual will
cause cancer to develop)
Dedi±
erentiation (loss oF specialized characteristics)
Loss oF contact inhibition
Ability to induce local blood vessel Formation (angiogenesis)
Invasiveness
Ability to metastasize (spread)
Normal cells
(with hairlike cilia)
Cancer cells
FIGURE 3.38
A cancer cell is rounder and less specialized
than surrounding healthy cells. It secretes biochemicals that cut
through nearby tissue (invasiveness) and other biochemicals that
stimulate extension oF blood vessels that nurture the tumor’s growth
(angiogenesis) (2,200×).
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