The limited capacity of all normal human and other animal cells to reproduce and function. The gradual decline in normal physiological function of the cells is referred to as aging or senescence. The aging process ends with the death of individual cells and then, generally, the whole animal. Aging occurs in all animals, except those that do not reach a fixed body size such as some tortoises and sharks, sturgeon, and several other kinds of fishes. These animals die as the result of accidents or disease, but losses in normal physiological function do not seem to occur. Examples of cells that do not age are those composing the germ plasm (sex cells) and many kinds of cancer cells. These cells are presumed to be immortal.
Although cultured normal human and other animal cells are mortal, they can be converted to a state of immortality. The conversion can be produced in human cells by the SV40 virus and in other animal cells by other viruses, chemicals, and irradiation. This conversion from mortality to immortality is called transformation, and is characterized by the acquisition of many profoundly abnormal cell properties, including changes in chromosome number and form, and the ability of the cells to grow unattached to a solid surface. These changes, and many more, are characteristic of most cancer cells. See also Cancer (medicine); Oncology; Tumor viruses.
