Essentials of Anatomy and Physiology

by chemicals from the adjacent damaged tissue. Or the
dividing cells may be stem cells that are among the
specialized cells. (A region of the brain called the hip-
pocampus, which is necessary to form new memories,
seems to have cells capable of division.) At present we
do not have definitive knowledge, but we do know that
for most people with heart damage or central nervous
system injury, mitosis does not take place, or not suf-
ficiently enough to replace the cells that have died and
preserve or restore normal functioning of the organ.

Research is continuing, and may eventually find the

stimulus necessary to produce extended mitosis that

would bring about true tissue repair.

MEIOSIS

Meiosisis a more complex process of cell division that

results in the formation of gametes, which are egg

and sperm cells. In meiosis, one cell with the diploid

number of chromosomes divides twice to form four

Cells 61

BOX3–3 ABNORMAL CELLULAR FUNCTIONING—CANCER

the trigger is believed to be infection with certain

viruses that cause cellular mutations. Carriers of

hepatitis B virus, for example, are more likely to

develop primary liver cancer than are people who

have never been exposed to this virus. Research has

discovered two genes, one on chromosome 2 and

the other on chromosome 3, that contribute to a

certain form of colon cancer. Both of these genes

are the codes for proteins that correct the “mis-

takes” that may occur when the new DNA is syn-

thesized. When these repair proteins do not

function properly, the mistakes (mutations) in the

DNA lead to the synthesis of yet other faulty pro-

teins that impair the functioning of the cell and pre-

dispose it to becoming malignant.

Once cells have become malignant, their func-

tioning cannot return to normal, and though the

immune system will often destroy such cells, some-

times it does not, especially as we get older.

Therefore, the treatments for cancer are directed at

removing or destroying the abnormal cells. Surgery

to remove tumors, radiation to destroy cells, and

chemotherapyto stop cell division or interfere

with other aspects of cell metabolism are all aspects

of cancer treatment.

New chemotherapy drugs are becoming more

specific, with very precise targets. For example, the

cells of several types of solid-tumor cancers have

been found to have mutations in the gene for the

cell membrane receptor for a natural growth factor

(epidermal growth factor receptor, or EGFR). These

altered receptors, when triggered by their usual

growth factor, then cause the cell to divide uncon-

trollably, an abnormal response. Medications that

target only these altered receptors have already

been developed for some forms of lung cancer and

breast cancer. Not only do they show promise in

treating the cancer, they do not have the side

effects of other forms of chemotherapy.

There are more than 200 different types of cancer,

all of which are characterized by abnormal cellular

functioning. Normally, our cells undergo mitosis

only when necessary and stop when appropriate. A

cut in the skin, for example, is repaired by mitosis,

usually without formation of excess tissue. The new

cells fill in the damaged area, and mitosis slows

when the cells make contact with surrounding cells.

This is called contact inhibition, which limits the

new tissue to just what is needed. Malignant

(cancer) cells, however, are characterized by uncon-

trolled cell division. Our cells are genetically pro-

grammed to have particular life spans and to divide

or die. One gene is known to act as a brake on cell

division; another gene enables cells to live indefi-

nitely, beyond their normal life span, and to keep

dividing. Any imbalance in the activity of these

genes may lead to abnormal cell division. Such cells

are not inhibited by contact with other cells, keep

dividing, and tend to spread.

A malignant tumor begins in a primary site such

as the colon, then may spread or metastasize. Often

the malignant cells are carried by the lymph or

blood to other organs such as the liver, where sec-

ondary tumors develop. Metastasisis characteris-

tic only of malignant cells; benigntumors do not

metastasize but remain localized in their primary

site.

What causes normal cells to become malignant?

At present, we have only partial answers. A malig-

nant cell is created by a mutation, a genetic

change that brings about abnormal cell functions

or responses and often leads to a series of muta-

tions. Environmental substances that cause muta-

tions are called carcinogens. One example is the

tar found in cigarette smoke, which is definitely a

cause of lung cancer. Ultraviolet light may also

cause mutations, especially in skin that is overex-

posed to sunlight. For a few specific kinds of cancer,