Cancerous Cell Growth and Development
During the development of cancer, the normal balance between cell division and cell loss is disrupted. The malignant cells divide far faster than new cells are needed. Since each division of a malignant cell results in two newly formed cells that retain their capacity to divide, there is an overall increase in the total number of dividing cells.
This gradual increase in the number of dividing cells creates the mass of tissue called a tumor or
. If there is no signal for apoptosis (cell suicide), the tumor will continue to grow as long as its blood supply remains intact. The rate at which a tumor grows is determined by the rate of cell division. Rapid cell division produces a rapidly growing (and generally more aggressive) tumor. A slower rate of cell division produces a slower growing (and generally less aggressive) tumor. Image 1 illustrates the process of tumor development.
|Image 1: Tumor development
Characteristics of Cancer Cells
All malignant cells share three characteristics:
Normal cell structure includes one
, and a clearly defined cell membrane and cellular scaffolding (
). 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.
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