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.