Cellular Aging: Telomeres

Aging is a complex process that occurs on multiple levels. The end result of aging is that life span is limited in multicellular organisms. The cells that make up multicellular organisms also have limited life spans. The limitation on cellular life span is comprised of two parts: (1) cells become unable to continue dividing but remain metabolically active, and (2) at some future time cell death occurs. Many cells in the human body are continually undergoing cellular division. Cellular division is a normal condition of certain tissues; examples include hair growth, the sloughing off of skin every several days, and the complete turnover and replacement of the cells of the immune systems every few months. In some instances, cellular division occurs in order to heal damaged tissues. Thus, having a limited number of cellular divisions available could contribute to aging by slowing down processes such as wound healing, as well as affecting general tissue maintenance.

In the 1960s, Leonard Hayflick first noted that human cells undergo a limited number of divisions when placed in culture. Furthermore, he noted that the number of divisions cells undergo is related to the number of prior divisions undergone by the cells. This observation suggested the existence of an intracellular clock that marked the division history of each cell. In addition, it suggested that once a predetermined number of divisions has occurred, a signal (or signals) is generated that prevents the cell from undergoing further divisions. The timing mechanisms underlying and regulating this process remained elusive until the end of the twentieth century. The first of these clocks to be identified and characterized, the telomere, is active in several human cell types.