Apoptosis is a form of programmed cell death. It selects certain cells to die during prenatal development, particularly in the thymus and brain. An apoptotic cell rounds up, the cell membrane forms blebs, the nuclear membrane breaks down, chromatin condenses, and DNA is cut into many equal-sized pieces. Stem cells actively divide, replenishing tissues. Different cell populations include specific proportions of cells in different stages of the cell cycle.
Cancer can result from excess cell division or deficient apoptosis.
A cancer cell divides more often or more times than surrounding cells, has an altered surface, lacks specialization, and divides to yield other cancer cells. A malignant tumor infiltrates nearby tissues and metastasizes if it reaches the bloodstream. Cancer can result from an overexpressed oncogene or an inactivated tumor suppressor gene and may be sensitive to environmental triggers. The McGraw-Hill Companies. Any use is subject to the Terms of Use and Privacy Policy .