We hear a lot about cancer, but what exactly are cancer cells? How do cancer cells differ from normal cells in our body?
Cancer Cell DefinitionA cancer cell is a cell that has achieved a sort of immortality. Unlike normal cells that stop growing at a certain point, cancer cells continue to divide out of control. And unlike normal cells that remain in the region where they began, cancer cells have the ability to both invade nearby tissues and spread to distant regions of the body. Below, several differences between cancerous cells and normal cells will be discussed.
Types of Cancer CellsThere are as many types of cancer cells as there are types of cancer. Of the hundred plus types of cancer, each is named for the type of cancer cells in which it began. And just as cancers may behave differently from one another, not all cancer cells behave the same.
How Do Cancer Cells Start?Cancer cells are usually formed after a series of mutations cause them to become increasingly abnormal. These mutations are either inherited, or more often, caused by carcinogens (cancer causing substances) in our environment. That cancer is caused by not one but several mutations explains why cancer is more common in older people and why it is often multifactorial (meaning there are several factors that work together to cause cancer.) It also helps explain a genetic predisposition to cancer. A genetic predisposition does not mean you will get cancer, but, simplistically, if a few mutations are already in place, it will likely take fewer acquired mutations for a cell to become cancerous.
Cancer Cells vs Normal CellsThere are many important differences between cancer cells and normal cells. Some of these include:
- Growth – Normal cells grow as a part of growth and development, or to repair injured tissue. Cancer cells continue to grow (reproduce) even when further cells are not needed. Cancer cells also fail to listen to signals that tell them to stop growing or commit cell suicide (apoptosis) when the cells become old or damaged.
- Ability to invade nearby tissues – Normal cells respond to signals from other cells which tell them they have reached a boundary. Cancer cells do not respond to these signals, and extend into nearby tissues often with finger-like projections. This is one reason why it is difficult at times to surgically remove a cancerous tumor.
- Ability to spread (metastasize) to other regions of the body – Normal cells make substances called adhesion molecules that cause them to stick to nearby cells. Cancer cells, lacking the stickiness caused by these adhesion molecules can break free and float to other regions of the body. They may travel to nearby tissue, or through the bloodstream and lymphatic system to areas of the body far from the original cancer cell – for example a lung cancer cell may travel (metastasize) to the lymph nodes, to the brain, or to the bones.
- Immortality – Normal cells, like humans, have a lifespan. When they reach a certain age, they die. Cancer cells, in contrast, have developed a way to “defy” death. On the end of our chromosomes is a structure known as a telomere. Every time a cell divides, its telomeres becomes shorter. When the telomeres become short enough, the cells dies. Cancer cells have figured out a way to restore their telomeres so that they don’t continue to shorten as the cell divides, thus, in a way, making them immortal.
Why Doesn’t the Body Recognize Cancer Cells as Abnormal and Destroy Them?A good question is, "Why don’t our bodies recognize and remove cancer cells as they would, say a bacteria or virus?" The answer is that most cancer cells are indeed detected and removed by our immune systems. Cells in our immune cells called natural killer cells have the job of finding cells that have become abnormal so that they can be removed by other cells in our immune system. Cancer cells remain alive either by evading detection or by inactivating the immune cells that come to the scene. A lot of research is in progress looking at ways to get the body to again recognize cancerous cells.
How do Cancer Cells Differ From Precancerous Cells?Precancerous cells may look abnormal and similar to cancer cells, but are distinguished from cancer cells by their behavior. Unlike cancer cells, precancerous cells do not have the ability to spread (metastasize) to other regions of the body.
Final Thoughts on Cancer Cells – An AnalogyAn analogy to describe cancer cells has been that of a car. The growth of the cells can be pictured as a car that has the accelerator stuck down. At the same time, the brakes don’t work (the cells doesn’t respond to tumor suppressor proteins.)
We can take this analogy a step further. The invasion of cancer cells can be viewed as a car breaking through a gate into a gated community. Normal cells respond to signals from neighboring cells that say “this is my boundary, stay out.” Cancer cells are antisocial in other ways as well. As they “gang” up with other cancer cells, all of which are becoming more immature in their actions over time (due to rapid division), they spread out and invade other communities as well.
But just as crime hasn’t overridden the United States, there are many police officers (checkpoints) that keep the majority of cells in the body in line.
It is actually very difficult for a normal cell to become a cancer cell. It has to be abnormal in ways that facilitate growth, inhibit repair and death, ignore signals from neighbors, and achieve a form of immortality. This is why cancer isn’t caused by a single mutation, but rather by a series of mutations. But considering that a billion cells in our bodies divide every day, something is bound to go wrong (mutations occur) once in awhile. And they do, for an estimated 1.6 million people in the United States each year.
American Cancer Society. Testing Biopsy and Cytology Specimens for Cancer. What do doctors look for under the microscope? Updated 03/24/10. http://www.cancer.org/treatment/understandingyourdiagnosis/examsandtestdescriptions/testingbiopsyandcytologyspecimensforcancer/testing-biopsy-and-cytology-specimens-for-cancer-what-doctors-look-for
DeBaradinis, R. et al. The biology of cancer: metabolic reprogramming fuels cell growth and proliferation. Cell Metabolism. 2008. 7(1):11-20.
National Cancer Institute. SEER Training Module. Cell Biology of Cancer. Accessed 01/17/13. http://training.seer.cancer.gov/disease/cancer/biology/
National Cancer Institute. What is Cancer? Updated 02/06/12. http://www.cancer.gov/cancertopics/cancerlibrary/what-is-cancer