The human body is a complex and
wondrous living machine 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