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