Also, 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.
This article explains how cancer cells develop, some of the ways in which cancer cells differ from normal cells, and why the body may not recognize cancer cells and destroy them as it does other “foreign” cells.
Types
There are as many types of cancer cells as there are types of cancer. Of the hundred-plus types of cancer, most are named for the type of cancer cells in which the disease began. For example:
Carcinomas are cancers that arise in epithelial cells that line body cavities. Sarcomas are cancers that arise in mesenchymal cells in bones, muscles, blood vessels, and other tissues. Leukemias, lymphomas, and myeloma are blood-related cancers that are arise from the bone marrow (leukemias and multiple myelomas) or the lymphoid tissues (lymphomas). They are “fed” by nutrients in the bloodstream and lymph fluid such that they don’t need to form tumors.
Just as cancers may behave differently from one another, not all cancer cells behave the same way.
How Do They Start?
Cancer cells appear through a series of genetic and epigenetic (or environment-induced) changes. Some of these changes may be inherited or, more often, caused by carcinogens (cancer-causing substances) in the environment.
In general, solid tumors contain multiple mutations (also known as variants, which represent a change from the original). Interestingly, the metastatic process that is the main culprit for the high mortality of advanced cancers is thought to be caused mostly by epigenetic changes as no specific genetic alterations have been found in metastases.
It helps to explain a genetic predisposition to cancer. A genetic predisposition does not mean you will get cancer, but if a few mutations are already in place, it will likely take fewer acquired mutations for a cell to become cancerous.
The process of normal cells becoming cancer often goes through stages in which the cell becomes progressively more abnormal in appearance. These stages may include hyperplasia (enlarged) and dysplasia (growing abnormally) before cancer.
Sometimes, this process is described as differentiation. Early on, a cell may look much like normal cells of that organ or tissue. As the progression continues, the cell becomes increasingly undifferentiated. This is why sometimes the original source of cancer cannot be determined.
What Makes Them Divide and Grow
A cancer cell can have thousands of mutations, but only a certain number of these genetic changes in cancer cells cause cancer to divide and grow. Mutations that result in the growth of cancer cells are referred to as “driver mutations,” whereas other mutations are considered “passenger mutations.”
Normal genes that help cells grow, called proto-oncogens, can become “oncogenes” (or those with the potential to cause cancer) when mutated and code for proteins that drive the growth of cancer. By contrast, tumor suppressor genes are genes within the cell that tell cells to slow down and stop growing and repair damaged DNA. And they tell cells when to die.
Cancer Cells vs. Normal Cells
There are many important differences between cancer cells and normal cells. Some of these include:
Growth: Normal cells grow during development stages, such as during childhood, 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 can be difficult 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. Some 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 (or metastasize) to the lymph nodes, brain, liver, or bones. Immortality: Most normal cells, like humans, have a limited 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 become shorter. When the telomeres become short enough, the cells die. Cancer cells have figured out a way to restore their telomeres so that they don’t continue to shorten as the cell divides. In this way, they become immortal.
The ability to invade and metastasize is very important in differentiating a cancer cell from a normal healthy cell, but there are many other important distinctions as well:
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 they can be removed by other cells in our immune system. Cancer cells remain alive either by evading detection (they disguise themselves in different ways) or by inactivating the immune cells that come to the scene.
The ability of the immune system to recognize and eliminate cancer cells is thought to be responsible for the uncommon but well-documented phenomena of some cancers going away without treatment (like the spontaneous remission of cancer.) This process also lies at the crux of the new field of cancer treatment known as immunotherapy.
Cancer Cells Keep Changing
Once a cancer has formed, the cells don’t remain the same. Rather, continued mutations may occur. This is why resistance develops to chemotherapy and targeted therapy drugs. The cancer cell develops a mutation that allows it to bypass the damaging effects of these treatments.
That cancer cells’ change is very important in treatment. For example, a breast cancer that is estrogen-receptor positive may be estrogen-receptor negative when it recurs or spreads. It also helps explain why cancer cells in different parts of a tumor may be different. This is referred to as “heterogenicity” and is important in diagnosis and treatment.
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.
An often-confusing condition is that of carcinoma-in-situ (CIS.) It consists of cells with the abnormal changes found in cancer cells. But since they have not spread beyond their original location (or technically, have not gone beyond something called the basement membrane), they are not technically cancer. Since CIS can turn into cancer, it is usually treated as early cancer.
Consider an Analogy
Many physicians liken cancer cells to a car with its accelerator stuck in the down position. At the same time, the brakes don’t work (meaning, the cells doesn’t respond to tumor suppressor proteins.)
You can take this analogy one step further. The invasion of cancer cells can be viewed as a car breaking through a gate in front of a gated community. Normal cells respond to signals from neighboring cells that say, “This is my boundary; stay out.”
But cancer cells are lawbreakers. They join other cancer cells and spread out to invade other communities, like crime. But as much as it threatens to, crime hasn’t overtaken the United States, Similarly, there are many “police officers” (checkpoints) that keep the majority of cells in the body in line.
Believe it or nor, it’s actually very difficult for a normal cell to become a cancer cell. It has to be abnormal in ways that encourage 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 by a series of mutations.
Considering that a billion cells in our body divide every day, something is bound to go wrong at some point. And it does, for more than 1.6 million Americans every year. They are diagnosed with cancer.
Summary
There are as many types of cancer cells as there are types of cancer. Of the hundred-plus types of cancer, most are named for the type of cancer cells in which it began. Cancer cells appear through a series of genetic and environment-induced changes. The process of normal cells becoming cancer often goes through stages in which the cell becomes progressively more abnormal in appearance. But this is just the beginning. Cancer cells differ from normal cells in their ability to spread, invade nearby tissue, and “live” on in perpetuity. They can accomplish this by evading detection (they disguise themselves in different ways) or by inactivating the immune cells that come to the rescue. Once a cancer has formed, the cells don’t remain the same. Rather, continued mutations may occur. This is why resistance develops to chemotherapy and targeted therapy drugs.
A Word From Verywell
For a disease that has such frightening connotations, cancer can be fascinating—to study. Whatever your motivation for learning about cancer cells, remember that your healthcare provider can be an excellent resource. Ask them to recommend books or articles that will set you on the path to understanding what has long been one of the leading and most perplexing causes of death in the United States.
