Cell Cycle: Cell Division and Cancer

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Cells

Cell division and growth normally occur at approximately the same rate as cell death.

Cell Division and Cancer

Cell division and growth normally occur at approximately the same rate as cell death. However, when cell division and growth are higher than the cell death rate, tissues enlarge. A neoplasm or tumor is a mass of tissue produced by abnormal cell growth and divi-sion. A tumor is called benign when it remains within the epithelium or a capsule made of connective tissue. This type of tumor seldom becomes life threatening and can usually be surgically removed if it affects tis-sue function.

Malignant tumors spread into surrounding tis-sues in a process called invasion. The primary tumor may result in malignant cells traveling to other organs or tissue to establish secondary tumors. This process, called metastasis, is not easily controlled. Cancerdevel-ops (FIGURE 3-19) and exhibits mutations disrupting normal cell growth. Usually, all tumor cells are daugh-ter cells of just one malignant cell. Malignancy often occurs when a normal gene mutates. These modified genes are called oncogenes. Genes that promote cell division are called proto-oncogenes, and oncogenes are created by these genes. Oncogenes often code for the proteins controlling cell division.

Most cancers are caused by mutations of the genes inside somatic cells. This is linked to mutations occurring during cell division. A small ­number of errors occur while DNA is being replicated before cell division. A mutation is defined as a DNA sequence with replication errors inside it. Mutations may also be caused by chemicals, toxins, radiation, and viruses. Cancer most commonly develops in tis-sues that experience frequent cell divisions such as epithelial cells. Multiple mutations, occurring overmany cell generations, result in cancer. This is why older people more commonly develop cancer than younger people.

Cancer cells change shape as they grow and grad-uallyresemble normal cells less and less. Tumor cells escape the primary tumor to invade the surrounding tissue—this is the manner in which metastasis begins. If tumor cells penetrate blood vessels, they circulate throughout the body. If they enter the lymphatic sys-tem, they accumulate in lymph nodes. The presence of tumor cells stimulates the growth of new blood vessels where the cells situate themselves. This supplies them with more nutrients and accelerates their growth and further metastasis.

As metastasis increases, organ function changes. Cancer cells grow and multiply by taking nutrients and space from normal cells, causing weight loss in most cancer patients as the normal cells deteriorate. When cancer cells compress vital organs or have replaced healthy cells in vital organs, death may occur. Cancer often begins where stem cells divide, because a greater chance of error occurs the more frequently that chromosomes are copied for cell division.

To prevent the development of cancer in cells, the body uses two major mechanisms. The first involves DNA repair enzymes, which detect and correct errors that occurred during replication. If these enzymes are made less effective because of their controlling genes becoming mutated, this first mechanism of fighting against cancer cannot be effective. The second mechanism is apoptosis, which is a self-destructive process that destroys cells containing abnormal DNA. There-fore, mutated cells are made to self-destruct before cancer can develop. Apoptosis also occurs in normal cells that have a limited life span. However, mutations of genes that influence apoptosis may result in per-sistently mutated cells, which can continue to divide and proliferate.

Tumor suppressor genes are normal genes thatslow or even stop cell division. They can be affected by mutations that cause them to have reduced actions. In human cancer cells, many types of oncogenes and altered tumor suppressor genes have been identified. Cancer therapy focuses on the confinement­ of malignant­ cells and then their destruction. Cancerous tissue is killed by lasers, X-rays, and drugs (­chemotherapy) and removed by surgery. However, many cancers cannot be com-pletely destroyed. Some of the adverse effects of cancer therapies include the destruction of nor-mal cells in rapidly growing tissues­ such as bone ­marrow. This can lead to anemia, because of the reduced numbers of red blood cells.

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