Cell death is a vital and common occurrence. In humans, some 10 billion new cells may form and an equal number die in a single day. Biologists recognize two general categories of cell death, which include genetically programmed death and death resulting from external forces (necrosis).
Genetically programmed cell death is necessary for replacing cells that are old, worn, or damaged; for sculpting the embryo during development; and for ridding the body of diseased cells. Toward the end of the twentieth century biologists recognized several mechanisms by which cell death could occur. In apoptosis, the most common form of normal cell death, a series of enzyme-mediated events leads to cell dehydration, outward ballooning and rupture of the weakened cell membrane, shrinking and fragmentation of the nucleus, and dissolution of the cell. By a different mechanism some cells generate special enzymes that "cut" cellular components like scissors (known as autoschizis, or "self-cutting"). Damaged cells that will become necrotic may lose the ability to control water transport across the membrane, resulting in swelling from excess fluid intake and disruption of protein structure (oncosis).
Programmed cell death is an important component of embryonic development and eliminates cells that are no longer needed. These include, for example, the cells between what will become fingers, or cells making up the embryo's original fish-like circulatory system as adult blood vessels form. Coordinate processes are called "cell determination," which involves a cell line becoming progressively genetically restricted in its developmental potential. For example, a cell line might become limited to becoming a white blood cell, thus losing the ability to become a liver cell. Cell differentiation occurs when cells take on specific structure and functions that make them visibly different from other cells (e.g., becoming neurons as opposed to liver epithelium).
All life is immortal in the sense that every cell is descendent from a continuous lineage dating back to the first nucleated cells 1.5 billion years ago. Life has been propagated through a repeating process of gamete (egg and sperm) formation by meiotic cell division (which creates genetic diversity by blending maternal and paternal genes), fertilization, and the development of the fertilized egg into a new multicellular organism that produces new gametes.
Can individual cells or cell lines, however, become immortal? This may be possible. HeLa cells (tumor cells from a patient named Henrietta Lack) have been kept alive and dividing in tissue culture for research purposes since 1951. But normal cells have a limit to the number of times they can divide, which is approximately fifty cell divisions (known as the Hayflick limit). The key to cell immortality seems to be the tips of the chromosomes, or telomeres, that protect the ends from degradation or fusion. Telomeres consist of a repeating sequence of DNA nucleotides. They shorten with each replication so that after some fifty divisions replication is no longer possible. An enzyme called "telomerase" adds these sequences to the telomere and extends the Hayflick limit. However, this enzyme is not very abundant in normal cells. When the biologists Andrea G. Bodnar and colleagues introduced cloned telomerase genes into cells, the telomeres were lengthened and the Hayflick limit for the cells greatly extended, suggesting the potential for cellular immortality.
Bodnar, Andrea G., et al. "Extension of Life Span by Introduction of Telomerase into Normal Human Cells." Science 279 (1998):349–352.
Darzynkiewics, Zbigniew, et al. "Cytometry in Cell Necrobiology: Analysis of Apoptosis and Accidental Cell Death (Necrosis)." Cytometry 27 (1997):1–20.
Raloff, Janet. "Coming to Terms with Death: Accurate Descriptions of a Cell's Demise May Offer Clues to Diseases and Treatments." Science News 159, no. 24 (2001):378–380.
ALFRED R. MARTIN