Imagine a healthy tissue containing thousands of cells. Each cell serves the greater good, which is the continuation of a person's life. Each cell is programmed so that when the cell is old or no longer needed, it dies a peaceful and timely death. This death is called apoptosis. All cells are in communication, which allows for the smooth repair and replacement of tissues and other aspects of cell behavior. Communication takes place either indi rectly, via exchange of messenger compounds such as hormones and growth factors, or directly, via cell-to-cell contact. Contact allows cells to respond to the "feel" of neighboring cells, via cell adhesion molecules, and to exchange messenger molecules through cell-to-cell portals called gap junctions. With the help of proper communication, appropriate cells proliferate when new cells are needed, and when enough new cells have been produced, cell division stops.
Cancer cells are the descendants of a normal cell in which something has gone wrong. In this normal cell, some kind of internal or external stress causes a mix-up in its genetic code (its DNA). This event is said to "initiate" the cell to a precancerous state. After its DNA has been damaged, the cell withdraws from close communication with its neighboring cells. Interrupted cell-to-cell communication is a common result of DNA damage or other forms of cellular damage. Separated from the regulatory controls of its community, it is now at the mercy of its environment. Let us say that the environment around this cell contains a promoting agent, which is a compound that stimulates cell proliferation. In response to the promoting agent, this precancerous cell divides to produce daughter cells, and these daughter cells divide to produce more daughter cells, and so on. All are proliferating only in response to the promoting agent. The promoting agent may be a chemical foreign to the body, or it could come from a natural process such as inflammation. One day, the worst occurs. The genetic instabilities passed down through the generations finally result in one cell that becomes capable of self-stimulation, and on this day an autonomous cancer cell is born. This cell no longer requires the promoting agent to stimulate its proliferation. The role of the promoting agent is made obsolete by the cell's ability to make proteins such as growth factors that stimulate proliferation.
This original cancer cell divides to produce daughter cells, these cells also divide, and soon there is a population of cancer cells. As they divide, they develop malignant characteristics, such as the ability to invade and metastasize. They also develop other characteristics that help assure survival, for example, the ability to evade the immune system, to mutate when faced with adverse conditions, and to induce the growth of new blood vessels through the process called angiogenesis. The development of these characteristics marks the third stage in carcinogenesis, the first two stages being initiation and promotion, respectively. In this book, I use the term progression to refer to both the third stage of carcino-genesis proper and to the entire postpromotion period of the cancer's life. This correctly implies that progression is an ongoing, evolving process.
Compared to normal cells, cancer cells have lost touch with their neighboring cells, their community purpose, and even largely with one another. They are a race of self-serving, easily adaptable cells, whose proliferation continues with the slightest provocation. They use more than their fair share of resources, live longer than their fair share of time, and produce more than their share of offspring. In short, they exhibit the two deadly characteristics of cancer: uncontrolled proliferation and uncontrolled spread.
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