Gene-Directed Programmed Cell Death
Programmed cell death, including apoptosis, is gene-directed. "The word comes from two Greek words, apo- and ptosis-, and the p is silent," declares Jonathan C. Busser, a researcher in the department of neurology and neurosurgery at Case Western Reserve University School of Medicine. "Apo" means "separate from" and "ptosis" means "fall from"--a description of cells that naturally, and without any inflammatory fanfare, die as part of normal development, he explains.
The steps of apoptosis are distinctive. The cell forms a tight sphere and its membrane undulates, resulting in bulges called blebs. The nuclear membrane breaks, and endonucleases clip chromosomes where the DNA peeks out from protective proteins. This occurs at 180-base intervals, so the DNA pieces are all the same size. Then the cell fragments, with enough membrane sequestering toxic cell contents to prevent inflammation at the site. Finally, nearby cells consume the remains. (In contrast to this process is necrosis, a nonprogrammed form of cell death that is a response to injury, in which the cell swells and bursts, causing inflammation.)
The pieces of the cellular death machinery are present in the cytoplasm, proven by the fact that cells whose nuclei are removed can still undergo apoptosis. A hypothesized "death signal" activates the process. Apoptosis is so fast that researchers often can't detect it, let alone sort out the sequence of events. "Once it starts, apoptosis probably takes from a few minutes to an hour," says Douglas Green, head of the division of cellular immunology at the La Jolla Institute for Allergy and Immunology in California.
On a cellular level, apoptotic cells are visualized with vital dyes and electron microscopy. On a molecular level, electrophoresis gels are used to display the telltale same-sized DNA pieces, which resemble ladders. Alternatively, the DNA pieces can be detected by labeling their 3_ ends with a biotinylated thymine analog. "Cells that stain brightly are the ones with large numbers of 3_ ends. If there's no bright stain, there's no apoptosis," comments Busser.
Apoptosis as part of normal development is a strategy to select certain cells for survival, sculpting a tissue's specificity. In a vertebrate embryo's limb, apoptosis carves fingers from webbing. In the developing brain it leaves behind only certain neural connections, and in the fetal thymus allows only T cells with "self" surfaces to complete development.
Later in life, apoptosis protects. Consider sunburn. A cell whose DNA is damaged by ultraviolet radiation in sunlight is either repaired or jettisoned via apoptosis--peeling (A. Ziegler et al., Nature, 372:773-6, 1994). "Such controls ensure that any one mutated cell cannot proliferate. Without this, tumors would be incredibly common," Green explains.
Developmental biologists have long been familiar with cell death in carving a vertebrate's digits and in insect metamorphosis. But...