Cell proliferation and cell death occur throughout the life of multicellular organisms. Animal development begins with the rapid proliferation of embryonic cells, which then differentiate to produce the many specialized types of cells that make up adult tissues and organs. Whereas the nematode C. elegans consists of only 959 somatic cells, humans possess a total of approximately 1014 cells, consisting of more than 200 differentiated cell types. Starting from only a single cell-the fertilized egg-all the diverse cell types of the body are produced and organized into tissues and organs. Cell proliferation is then needed throughout life to replace cells that have died. Many adult tissues contain stem cells that are able to proliferate and differentiate as required for tissue maintenance. The ability of stem cells to differentiate into a wide variety of cell types has generated enormous interest in the potential clinical applications of using these cells to replace damaged tissues.
Although cells can die as a result of unpredictable traumatic events, such as exposure to toxic chemicals, most cell deaths in multicellular organisms occur by a normal physiological process of programmed cell death, which plays a key role both in embryonic development and in adult tissues. Abnormalities of cell death are associated with a wide variety of illnesses, including cancer, autoimmune disease, and neurodegenerative disorders, such as Parkinson's and Alzheimer's disease. The mechanisms and regulation of cell death as well as cell renewal have therefore become areas of research at the forefront of biology and medicine.