What Is Cancer?, What Causes Cancer?, Cancer And Aging
Complex multicellular organisms contain two basic classes of cells: mitotic and postmitotic. Postmitotic cells cannot divide, although they may function throughout adult life. Examples of postmitotic cells include mature neurons, adipocytes (fat cells), and mature muscle cells. Mitotic cells, by contrast, retain the ability to divide throughout life. Mitotic cells may divide continually, or they may divide only when there is a need for cell replacement or tissue repair. Mitotic cells include the differentiated (specialized) cells of epithelial tissues, such as skin, liver, colon, lung, breast, and prostate. They also include certain cells of the immune system, such as T and B lymphocytes, and cells that support epithelial and postmitotic cells, such as fibroblasts and glia. Cancers never arise from postmitotic cells, but only from mitotic cells.
Cancers in children and young adults tend to be caused by defects in development or tissue maturation. Early-life cancers include certain cancers of the developing immune system (leukemias), retinoblastoma (cancer of the developing retina), and osteosarcoma (bone cancer). Early-life cancers tend to arise from precursors to postmitotic cells (i.e., a neuroblastoma tumor is composed of precursors to neurons), or mitotic cells that support epithelial or postmitotic cells (for example, fibrosarcoma or glioblastoma tumors are composed of fibroblasts and glial cells, respectively).
By contrast, most cancers that arise during middle and old age derive from epithelial cells and, to a lesser extent, the immune system. Epithelial tissues such as skin, stomach, and colon are composed of cells that are constantly sloughed off and replaced, and therefore contain cells that proliferate constantly throughout life. Other tissues show periodic or relatively slow proliferation. Examples include the breast, where epithelial cells proliferate with each estrus cycle, and the mature immune system, where cells proliferate in response to specific antigens. Tissues that show constant or periodic cell proliferation are particularly prone to developing tumors later in life. Tissues in which cells divide relatively infrequently are much less susceptible to developing into cancer with age. Examples include liver and kidney tissue, as well as cells that support and direct the functions of epithelial and postmitotic cells, such as fibroblasts and glia. However, chronic injury, toxicity, or infection can greatly increase tumor incidence in these tissues.
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