4
146
Given good nutrition, good circulation, and relatively infre-
quent wounds and infections, our tissues normally function ef-
ficiently through youth and middle age. But with increasing age,
epithelia thin and are more easily breached. Tissue repair is less
efficient, and bone, muscle, and nervous tissues begin to atro-
phy, particularly when a person is not physically active. ±ese
events are due partly to decreased circulatory efficiency, which
reduces delivery of nutrients to the tissues, but in some cases,
diet is a contributing factor. As income declines or as chew-
ing becomes more difficult, older people tend to eat soF foods,
which may be low in protein and vitamins. As a result, tissue
health suffers.
Another problem of aging tissues is the likelihood of DNA
mutations in the most actively mitotic cells, which increases the
risk of cancer (see
A Closer Look
on p. 145).
Check Your Understanding
23.
What are the names of the three embryonic germ layers?
24.
Which germ layer gives rise to the nervous system?
25.
Which two tissue types remain highly mitotic throughout
life?
For answers, see Appendix H.
As we have seen, body cells combine to form four discrete
tissue types: epithelial, connective, nervous, and muscle tissues.
±e cells making up each of these tissues share certain features
but are by no means identical. ±ey “belong” together because
they have basic functional similarities. ±e important concept
to carry away with you is that tissues, despite their unique abili-
ties, cooperate to keep the body safe, healthy, and whole.
Cancer Treatments
Most cancers are removed surgically
if possible. To destroy metastasized
cells, surgery is commonly followed
by X irradiation and/or treatment
with radioisotopes and chemotherapy
(treatment with cytotoxic drugs). Recently,
some oncologists have been using heat
therapy (a slight upward temperature
change) to make cancer cells more
vulnerable to chemotherapy or radiation.
Chemotherapy is beset with the problem
of resistance. Some cancer cells can eject
the drugs in tiny bubbles or flattened
vesicles dubbed exosomes, and these cells
proliferate, forming new tumors that are
resistant to chemotherapy. Furthermore,
anticancer drugs have unpleasant side
effects—nausea, vomiting, hair loss—
because they kill
all
rapidly dividing cells,
including normal tissue cells. The anticancer
drugs also can damage the brain, producing
a phenomenon called chemobrain—mental
fuzziness and memory loss reported by
many cancer patients. X rays also have side
effects because, in passing through the
body, they destroy healthy tissue as well as
cancer cells.
Promising New Therapies
Traditional cancer treatments—“cut, burn,
and poison”—are widely recognized
as crude and painful. Promising new
therapies focus on
Interrupting the signaling pathways
that fuel cancer growth.
Imatinib
(brand name Gleevec) incapacitates
a mutated enzyme that triggers
uncontrolled division of cells in two
rare blood and digestive system
cancers. Trastuzumab (Herceptin) is
used to treat breast cancer. These
drugs can provide a few extra weeks of
life, before their protective effects wear
off and the disease progresses again.
•■
Delivering treatments more precisely to
the cancer while sparing normal tissue.
One approach is to inject the patient
with tiny drug-coated metal beads;
then a powerful magnet positioned
over the cancer guides the beads to the
tumor. Or, a patient might take light-
sensitive drugs that are drawn naturally
into rapidly dividing cancer cells. Then,
exposure to certain frequencies of
laser light sets off reactions that kill the
malignant cells. Proton therapy delivers
highly targeted killing doses of protons
(radiation) with incredible precision
and effectiveness. Unlike X rays, which
pass through the cancer and onward
through the patient’s body, protons can
be slowed down and even directed to
stop in the neoplasm.
•■
Using genetically modified immune
cells to target cancer cells.
One
promising technique harvests a
patient’s most aggressive cancer-killing
immune cells (T lymphocytes), inserts
modified genes into them that make
them even more efficient, multiplies
the cells in the lab, and infuses them
back into the patient.
•■
Testing genotypes.
A few major
cancer centers are beginning to
genotype (test for genetic markers)
every patient’s tumor. The hope is
to match personalized markers with
drugs tailored to go after the tumor’s
genetic weak spot. For example,
taxol, quite successful in treating
breast and ovarian cancer, works only
against tumors with a specific genetic
makeup.
Other experimental treatments seek
to starve cancer cells by cutting off
their blood supply, fix defective tumor
suppressor genes and oncogenes,
destroy cancer cells with viruses, or
signal cancer cells to commit suicide by
apoptosis. A cancer vaccine (TRICOM)
contains genetically engineered viruses
carrying genes for a cancer protein called
carcinoembryonic antigen (CEA). When
CEA proteins are delivered into the
patient’s body, they stimulate an immune
response that orchestrates an attack on all
CEA-bearing cancer cells.
At present, about half of all cancer
cases are cured. Although average survival
rates have not increased, cancer patients’
quality of life has improved in the last
decade. We can offer better treatments
for cancer-associated pain, and antinausea
drugs and other helpful medicines can
soothe the side effects of chemotherapy.
A CLOSER LOOK
Cancer: The Intimate Enemy
(continued)
A polyp in the colon
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