Chapter 23 Cancer英文电子书.pdf

23 CANCER A blood smear from a person with acute myelogenous leukemia. The gigantic cells with irregularly shaped purple nuclei are leukemia cells. The small reddish-gray circular cells are normal red blood cells. [Margaret Cubberly/Phototake.] ancer causes about one-fifth of the deaths in the gens, which include certain chemicals and ultraviolet radia- United States each year. Worldwide, between 100 and tion. Cancer-causing mutations occur mostly in somatic cells, 350 of each 100,000 people die of cancer each year. C not in the germ-line cells, and somatic cell mutations are not Cancer is due to failures of the mechanisms that usually con- passed on to the next generation. In contrast, certain inher- trol the growth and proliferation of cells. During normal de- ited mutations, which are carried in the germ line, increase velopment and throughout adult life, intricate genetic the probability that cancer will occur at some time. In a de- control systems regulate the balance between cell birth and structive partnership, somatic mutations can combine with death in response to growth signals, growth-inhibiting sig- inherited mutations to cause cancer. nals, and death signals. Cell birth and death rates determine Thus the cancer-forming process, called oncogenesis or adult body size, and the rate of growth in reaching that size. tumorigenesis, is an interplay between genetics and the envi- In some adult tissues, cell proliferation occurs continuously