Normal Cells vs. Tumor Cells
Tumor cells vary from normal cells in several basic ways. First,
the division of normal cells is tightly regulated by special
cell signals. With tumor cells, it's as if the signals are no
longer produced or perhaps they are no longer received.
Research involving cells is often accomplished by removing the
cells from an individual and growing them in a sterile dish with
the nutrients required for their survival. Growing cells for
research use is termed "cell culture". Just by watching normal
cells in cell culture it is obvious that their division is
regulated by something. Normal cells in culture grow until the
bottom of their dish is carpeted with the cell. The layer is
only 1 cell thick. Once this density is reached, they stop
dividing because there is no more space. If one cell dies, an
adjacent one will divide to fill in the space. Additionally,
normal cells will divide a certain number of times after which
time, the division process halts. There are a certain
pre-determined number of generations that may be produced and
then there is no more dividing. Eventually, the entire culture
will die.
With tumor cells, it's a completely different story. Tumor cells
will divide over and over, time after time; forever if supplied
with nutrients. With enough time, tumor cells in culture will
become a piled up mess. They lack order to their growth. It is
as though tumor cells lose have lost the capacity to follow the
rules and they divide (proliferate) out of control.
A second major difference between normal cells and tumor cells
is that normal cells perform a special function or duty for the
body. Healthy cells have specialized behaviors and serve a
purpose. For example, lung cells have a specialized duty to
perform while cells of cardiac tissue have a very different one.
Normal cells taken from different tissues even have very
different appearances. Tumor cells have a different appearance
than normal cells taken from the tissue they are derived from.
This is due to the fact that they have lost their specialized
function.
Differentiation is the term given to describe the specialized
function a given cell has. Differentiation and proliferation are
closely tied together. In general, a cell that proliferates at a
high rate loses some of its specialized function. The problem
is, it really doesn't have time to perform a specific function
since its too busy dividing. Cells that perform a highly
specific function (i.e. differentiated) have a lower rate of
proliferation. Researchers are studying the possibility of
making tumor/cancer cells differentiated so they might lose
their ability to proliferate continuously. In theory, this would
cause the tumor to stop growing.