Normal cell structure includes one
chromosomes, and a clearly defined cell membrane and cellular scaffolding (
cytoskeleton). Cancer cells, on the other hand, may have two or three nuclei, abnormal or incorrect numbers of chromosomes, and poorly defined membranes and cytoskeletons. If their DNA is damaged, normal cells will also have the ability to repair themselves or to self-destruct through apoptosis, which effectively prevents them from becoming cancerous. Cancer cells tend to lose these capabilities.
Because of these structural abnormalities, cancer cells and the tissue they make up have a distinctive appearance under the microscope. Some of the traits doctors look for to identify a malignancy are:
- Large number of dividing cells
- Variation in nuclear size and shape
- Variation in cell size and shape
- Loss of specialized cell features
- Loss of normal tissue organization
- Poorly defined tumor boundary
- Unusual staining characteristics when treated with particular dyes or other preparations designed to highlight cellular components not easily seen with the microscope
Image 2 illustrates the characteristics of cancer cells.
Image 2: Characteristics of cancer cells
A process called
occurs when a cancer cell loses the distinctive features of the tissue of origin to such an extent that it starts to become indistinguishable from other cell types. In poorly differentiated prostate cancer, for example, doctors would find it difficult to identify a cancer cell as coming from the prostate gland. Generally speaking, the more dedifferentiated the cells, the more aggressive the cancer.
Tissue invasion refers to the direct migration and penetration of cancer cells into neighboring tissues. As you learned in the previous section, the growth of normal cells is controlled by contact inhibition, which ensures that when cells make contact with each other, they stop growing. Cancer cells do not follow the normal rules of contact inhibition and continue proliferating even when they make contact with other cells. This unchecked growth, combined with their ability to secrete enzymes capable of breaching confined spaces, allows cancer cells to invade and destroy adjacent normal tissues. This process is called tissue invasion.
The most deadly aspect of cancer is its ability to metastasize to a distant site in the body. It does this by draining into the
or by penetrating blood vessels that carry the cancer cells elsewhere in the body. The tumor from which these traveling cells break off is called the primary tumor. Primary tumors regularly shed cancer cells. Most of these cells die or are killed by the body’s immune system; but occasionally, one survives and lodges at a new, remote site.
Often, the first new site where a metastatic tumor will develop is in the lymph nodes in the region immediately adjacent to the primary tumor. For this reason, regional lymph nodes are often examined in an attempt to determine how far a tumor has spread. If regional nodes are not involved, chances are very good that the tumor has remained localized.
Unlike with lymphatic spread, when metastases are spread by tumor cells reaching the blood stream and being lodged at distant sites, there may be no local indication of the risk of distant spread. Certain tumors are more likely to spread by the blood stream than others, and this is probably related to a tumor's ability to encourage new blood vessel formation to feed the primary cancer site as well as metastatic sites.
If sufficient growth factors are present and the blood supply is adequate at the site, the cancer cell proliferates, creating a secondary tumor. Many tumors actually trigger the development of their own blood vessels, a process called
angiogenesis. See the Focus Box below to learn more about angiogenesis.
In most patients who die of cancer, it is the process of metastasis, not the primary tumor, which is to blame. Image 3 illustrates the processes of tissue invasion and metastasis.
Image 3: Tissue invasion and metastasis
Focus Box: What is Angiogenesis?
The normal, healthy body maintains a balance of angiogenesis modulators. However, in some serious diseases the body loses control over angiogenesis and the new blood vessels grow excessively. Excessive angiogenesis occurs when diseased cells produce abnormal amounts of angiogenic growth factors, overwhelming the effects of natural angiogenesis inhibitors. Excessive angiogenesis occurs in diseases such as cancer. The new blood vessels carry oxygen and nutrients to satisfy the cancer’s ravenous appetite for energy, harm nearby normal tissues, and allow tumor cells to proliferate at sites distant from the original tumor (metastases).