Cancer arises when cells fail to be restrained by the normal devices that stop them from surviving and dividing when conditions are unfavorable.
Animal tissues, especially in humans, renew and repair themselves regularly. Old cells die off and are replaced by new ones. This process works well when it functions correctly, but sometimes it fails. Cancer is a disease that results from an overgrowth of cells that survive and divide abnormally.
In healthy tissue, cells divide only when new cells are needed. The cell cycle is a biological process that includes cell growth, replication of DNA, mitosis, and cytokinesis. Specifically, this cyclic process begins with the G1 phase, followed by the S, G2, and M phases. The order in which the phases of this process occur is highly regulated by certain growth factors, such as steroids and hormones. That is, these growth factors control when a cell enters or exits each of these stages. When a cell reaches a stop point, also called a DNA-damage checkpoint, the cell cycle is stopped if there is damage to the DNA in need of repairing, for example, following environmental exposure to a substance that causes this damage. With cancer, not only is damaged DNA left unchecked, but this cell cycle process is disrupted, leading to the progression of uncontrolled cell division. During normal cell division, where cell growth is well controlled, each cell carries out specific functions related to its specialization: nerve cells transmit electrical signals, muscle cells contract and relax, and red blood cells take up and release oxygen. They also stay where they are needed: skin cells stay on the surface of the body, bone cells stay in the skeleton, and liver cells stay in the liver.
Cancer cells do not behave this way. There are two common features of cancer cells:
- They proliferate against constraints that prevent normal cells from dividing.
- They invade spaces that should be reserved for other cells.
Most cancer is said to occur in four stages. Stage I describes small, localized tumors. Stages II and III describe tumors that are growing and affecting surrounding tissues. Stage IV describes metastatic cancer. Some cancers, like breast cancer, also have a Stage 0, which is considered carcinoma in situ. This means the cancer is at a very early stage and has not grown beyond its initial location. Stage 0 is also very difficult to detect, as, a growth or lump may not have formed. This stage represents a small collection of abnormal, cancerous cells hidden within the normal surrounding tissue. Stage 0 cancer, if caught before progression, is often highly curable through complete excision.
Development of Cancer
|Stage I||The cancerous lesion has grown, but is small and localized. Other tissues are not yet involved. It has not spread to the lymph nodes.|
|Stage II||The cancer is growing and is beginning to affect (but has not grown deeply within) nearby tissues. One or more lymph nodes near the tumor are affected.|
|Stage III||The cancer is increasingly affecting nearby tissues. More distant lymph nodes may be affected, but it has not spread to other parts of the body.|
|Stage IV||The cancer has metastasized. Metastasis means that tissues or organs other than those near the original tumor are also affected. Cancer at this stage is considered advanced.|
Cancer cells are not like normal cells in several important ways:
- Cancer cells do not depend on signals from other cells for growth, division, and survival. Normal cells grow and divide only when there is room for them to do so. Cancer cells do not. Furthermore, most normal cells (excluding neurons and muscle cells, which, assuming no trauma or damage, last the lifetime of an organism) die off once they have completed their task, a process called apoptosis, or programmed cell death. Cancer cells continue to survive long after normal cells would have died.
- Cancer cells survive stresses that induce apoptosis in normal cells. This is especially true regarding damage to DNA. Cells whose DNA has become damaged usually undergo apoptosis, but cancer cells do not do this. Because of this, cancer cells have higher rates of DNA damage and mutations than normal cells.
- Cancer cells divide indefinitely. A healthy somatic cell, or body cell, has a limited number of divisions it can undergo before it must stop. Cancer cells continue to divide without ceasing. In fact, a line of cells derived from a cancerous tumor taken from a cancer patient named Henrietta Lacks has been dividing in culture since 1951.
- Cancer cells are invasive of surrounding tissues. This often happens because they lack cadherins, the proteins that anchor cells to one another. The absence of this anchor protein coupled with the ability to suppress apoptosis means that cancer cells can survive and proliferate in tissues where normal cells would die.
- Cancer cells have an accelerated metabolism, causing them to take up a disproportionately large share of nutrients, unlike normal cells, which they use to grow and divide unchecked.