like any machine our bodies are composed of smaller parts that work together

the various tasks that are performed each day

such as breathing eating and digesting food

and the movement of our blood are performed by specialized structures

known as

organs. Examples of organs include the lungs

stomach, heart and brain.

All of these organs and other body parts like our skin and muscles

are made of smaller living units called cells.

Our lungs are a good example

of how many cells work together to perform a specialized task

the cells in our lungs work together to allow us to take in oxygen from the air

and get rid of waste products such as carbon dioxide.

our lungs are composed of many millions of cells

working together to accomplish this task

cancer is the result of a long process

that begins when one of the cells in an organ or tissue

becomes damaged or altered in a way that causes it to break free

from the normal controls that allow our cells to work together in harmony.

A group of misbehaving cells

can cause the same kinds of problems in a body

that a defective part would cause in any other type of machine.

A normal cell will divide only when it receives a chemical signal telling it to

do so

these signals are interpreted in the nucleus

and the cells reproduce their genetic information and divide into two

identical daughter cells

through a process called my mitosis. Cancer cells do not obey this rule

and will divide even if they do not receive appropriate signals

in addition to the signals that normal cells receive telling them to divide

they are also told when to stop dividing. This prevents too many cells from being

made

in fact the cell division process

is a highly ordered process. This is a critical issue

in cancer because cancer cells do not obey

or require normal signals for division. This can lead to the formation of a

mass of cells

that piles up and may form a tumor. Also different from normal cells

is the ability of cancer cells to continue dividing indefinitely

An important point about cells

is that no matter what their job is in the body they all have the same general

structure.

the cells that make up our lungs, heart

or brain are all similar in their overall appearance

even though their jobs are quite different. just as the organs that the former made

a smaller structures

cells themselves are composed of smaller parts that help them perform their jobs.

these smaller structures are called organelles.

Of particular importance in cancer is the organelle known

as the nucleus. The nucleus can be thought of as the brains of a cell

it contains the information that acts as the blueprint for

each and every one of us just as a manual would contain instructions to

assemble a chair.

Specifically this information is contained within the chromosomes

that reside in the nucleus.

individual units have information are called genes.

At a chemical level genes are made of deoxyribonucleic acid

or DNA. All of our cells contain the same set of information

It is only how the information is used that makes them different.

For example cells in the lungs use different bits of the blueprint to do

their job

than the cells in our stomach. In cancer

changes to the DNA cause some of the genes to fail to perform

or to do their job in a way that causes problems for the affected

individual

In short all cancers are thought to result from changes to DNA

that alter critical genes and change the behavior of the affected cells.

If a change occurs to the nucleotide sequence

it is like having the letters of a word changed.

An alteration in a gene is called the mutation

How do all these changes occur? There are actually a number of different things

that can cause mutations

Examples include chemicals that can be swallowed or inhaled

Such as those found in chewing tobacco and cigarette smoke

and radiation from the Sun or artificial sources like

a tanning bed Sometimes mutations occur without

any known external cause They just happen.

Certain genes make products that lead cells to reproduce

This would be equivalent to the gasoline system in cars

The genes that are responsible for making cells divide

are known as Proto-Oncogenes. Changes in these normal genes

lead to the production of Oncogenes The result may be cells that divide in

the

absence of proper signals It is the equivalent

of a gas pedal that is stuck in the on position

Making a car go, even when no foot

is pushing down.

Genes whose products function as the equivalent of cellular breaks

also exist As a group these genes are known as tumor suppressors

Humans have two copies of each gene one inherited from each parent

If a single copy of a tumor suppressor is damaged

the other copy is usually able to stop the cell from behaving abnormally

This would be like losing either the front

or rear brakes of a car. The car may be damaged

but would still be able to stop and but if the second set of brakes is damaged

as well

the car would not be able to stop. Just as the cell would not be able to stop

dividing

if both copies of the tumor suppressor genes are damaged

The process by which tumors cause the body to provide them with nutrients

is known as angiogenesis Like the hungry plant in the Little Shop of Horrors

a growing tumors sends out signals that essentially say

feed me The messages from the tumors cause nearby blood vessels

to send over new extensions that deliver food and oxygen

Importantly, the blood vessels

also serve as a passageway for the movement of tumor cells to neighboring

and distant parts of the body. Spread of tumors to distant locations is of great

importance in cancer.

About ninety percent of the deaths due to cancer

involve tumors that have spread around the body. The movement of tumor cells to

other parts of the body

is known as metastasis. Metastasis is a complex process

During which cancer cells break off the original

or primary tumor and move through the body to form tumors at new locations

From the point of view of a cancer cell, this is a dangerous

and often unsuccessful process A trip through the body is full of hazards that

cause the death of most cells that begin the journey

even tough cancer cells. To begin the process

individual cells must break away from the tumor and invade nearby vessels

the cells crawl along the surface of other cells

and the fibrous stringy structures surrounding them

and then force their way in. Shown here

is the invasion of the blood supply. Once inside a blood vessel,

the cancer cells may parish from a variety of causes

Some cells die simply because they are unable to survive

floating around in the bloodstream.

Others may become damaged and die when they squeeze through tight spaces

or bump into the walls of the blood vessels. Still

other migrating cells may be recognized and destroyed

by cells of the immune system. How

and where the migrating cells stop is different for different cancer types

Once the tumor cells are no longer moving

they can begin the process of forming a new tumor

by leaving the blood vessel and beginning to reproduce in the new

location

This does not always occur

and cells that have made it this far may still die or fail to divide

If the new environment is suitable the newly-arrived cell will begin to grow

and a new tumor will develop

One way that the development of cancer is prevented

is via the death of defective cells A cell that becomes mutated or damaged

will first attempt to repair the damage. If that is not possible

the cells will commit the cellular version of suicide

An orderly process called apoptosis

leads to the breakdown of key cell parts and the death of the cell

Cancer cells lose this critical capability

and will continue to divide This can lead to the accumulation of cells

that can become more and more abnormal

Due to the high rate at which cancer cells accumulate mutations

a tumor that originally started as a single abnormal cell

is actually made up of many slightly different cells

they are all cancer cells and are similar to each other

but they may not all have the same sensitivity to any particular cancer

drug or treatment

When this mixed bag of cells is exposed to a drug,

most to the cells will probably die.

Those cancer cells that are resistant to the treatment

may reproduce to form a new tumor Importantly

this new tumor will be unlikely to respond to the same treatment

Their mutations make them invulnerable to that drug