A Cancer Primer

[Guest guest]

Last night I came across some brief comments on cancer in my physiology book (by Tortora). It helped give me a basic framework for understanding some of the discussions on this group, so I'm including here a few brief comments and definitions from the book. Zoe

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A cell has three possible destinies--to remain alive and functioning without dividing, to grow and divide, or to die.

The signals that tell a cell when to exist in the G0 [non-dividing] phase, when to divide, and when to die have been the subjects of intense and fruitful research during the past decade.

Cellular death is regulated. Throughout the lifetime of an organism, certain cells undergo apoptosis, an orderly, genetically programmed death. In apoptosis, a triggering agent from either outside or inside the cell causes "cell-suicide" genes to produce enzymes that damage the cell in several ways, including disrupting its cytoskeleton and nucleus. As a result, the cell shrinks and pulls away from neighboring cells. The DNA within the nucleus fragments, and the cytoplasm shrinks, although the plasma membrane remains intact. Phagocytes in the vicinity then ingest the dying cell. [iodine is believed to be important in apoptosis. This is probably one of the reasons iodine seems to be effective with various cancers.]

Apoptosis, a normal type of cell death, contrasts with necrosis, a pathological type of cell death that results from tissue injury.

Cancer is a group of diseases characterized by uncontrolled cell proliferation. When cells in a part of the body divide without control, the excess tissue that develops in called a tumor or neoplasm. The study of tumors is called oncology.

Tumors may be cancerous and often fatal, or they may be harmless. A cancerous neoplasm is called a malignant tumor or malignancy.

One property of most malignant tumors is their ability to undergo metastasis, the spread of cancerous cells to other parts of the body. A benign tumor is a neoplasm that does not metastasize. An example is a wart. A benign tumor may be surgically removed if it interferes with normal body function or becomes disfiguring.

Types of Cancer. The name of a cancer is derived from the type of tissue in which it develops. Most human cancers are carcinomas, malignant tumors that arise from epithelial cells. Epithelial cells cover body surfaces and line hollow organs, body cavities, and ducts. They also form glands. [There are four basic types of body tissues or cells: (1) epithelial tissue, (2) connective tissue, (3) muscle tissue, and (4) nervous tissue.]

Melanomas are cancerous growths of melanocytes (skin epithelial cells that produce the pigment melanin).

Sarcoma is a general term for any cancer arising from muscle cells or connective tissues. Connective tissue protects and supports the body and its organs. Various types of connective tissue bind the organs together, store energy reserves as fat, and help provide immunity to disease-causing organisms.

Leukemia is a cancer of blood-forming organs characterized by rapid growth of abnormal leukocytes (white blood cells).

Lymphoma is a malignant disease of lymphatic tissue--for example, of lymph nodes.

Growth and Spread of Cancer. Cells of malignant tumors duplicate rapidly and continuously. As the cancer grows, it begins to compete with normal tissues for space and nutrients. Eventually, the normal tissue decreases in size and dies. Some malignant cells may detach from the initial (primary) tumor and invade a body cavity or enter the blood or lymph, then circulate to and invade other body tissues, establishing secondary tumors.

The pain associated with cancer develops when the tumor presses on nerves or blocks a passageway in an organ so that secretions build up pressure.

Causes of Cancer. Several factors may trigger a normal cell to lose control and become cancerous. One cause is environmental agents: substances in the air we breathe, the water we drink, and the food we eat. A chemical agent or radiation that produces cancer is called a carcinogen. Carcinogens induce mutations, permanent structural changes in the DNA base sequence of a gene. The World Health Organization estimates that carcinogens are associated with 60-90% of all human cancers.

Carcinogenesis, the process by which cancer develops, is a multistep process in which as many as 10 distinct mutations may have to accumulate in a cell before it becomes cancerous. It may take years or decades to develop.

Although most cancers are thought to derive from a single abnormal cell, by the time a tumor reaches a clinically detectable size, it may contain a diverse population of abnormal cells. For example, some cancerous cells metastasize readily, and others do not. Some are sensitive to chemotherapy drugs and some are drug resistant. Because of differences in drug resistance, a single chemotherapeutic agent may destroy susceptible cells but permit resistant cells to proliferate.