Apoptosis is defined by its highly characteristic morphology, which is illustrated in Figure 1. Early changes include a reduction in cell volume (2,3) and loss of surface micro villi. Cytoskeletal changes result in the formation of protrusions on the surface of the cell, which are referred to as blebbs. These may break away as intact vesicular structures, giving rise to "apoptotic bodies" (4).
One of the most striking changes during apoptosis occurs within the nucleus. Fluorescence microscopy of a typical viable cell following staining with a DNA stain such as acridine orange, reveals a large spherical nucleus that may constitute most of the cellular volume. Often, the chromatin is highly diffuse, although occasionally it may be possible to see condensed chromosomes in mitotic cells. During apoptosis, the nucleus undergoes major changes. Initially, the chromatin condenses and marginates to the nuclear membrane, forming crescent or ring-shaped structures that exhibit intense fluorescence. Eventually, the chromatin collapses into two or more particles, which are often highly spherical. Again, the remainder of the cell will be devoid of chromatin and will be almost transparent in appearance. The shape of the cell also undergoes a highly characteristic change. While viable cells tend to be highly irregular in shape, on entry into apoptosis they become smooth-surfaced and in many cases almost spherical.
The changes in nuclear morphology coincide with the activation of a nuclease enzyme that cleaves chromatin first into 300 and/or 50-kbp fragments (5), and then ultimately into multiples of 200 bp (6). The first stage is believed to be responsible for the morphological changes described above. The second stage represents the cleavage of chromatin at the internucleosomal linker regions. This generates a striking ladder like pattern when DNA samples from apoptotic cells are subjected to DNA gel electro-phoresis. Together with condensation of chromatin, this so-called DNA ladder has become one of the hallmarks of apoptotic death.
Many cell types express a further enzymatic activity— that of the transglutaminase enzyme (7). This crosslinks proteins within the cell, generating a protein scaffold that is believed to hold the dead cell together, thus explaining the relative robustness of dead apoptotic cells as compared with cells that have undergone necrotic death.
These highly controlled changes appear to have a specific task: to provide a very clean and rapid method of eliminating dead cells, which is a vital consideration when one considers the extent of apoptotic cell death during, for example, embryogenesis. The cleavage of chromatin does not appear to be responsible, in itself, for the death of the cell. Instead, it has been argued that this ensures the complete destruction of the genetic material. It is suggested that this reduces the possibility of malignant transformation of surrounding cells by DNA from dying cells. The stabilization of the dead cell by the transglutaminase enzyme minimizes the probability of leakage of its contents onto surrounding cells. In vivo, surrounding cells and phagocytes engulf the dead cell before it can cause damage to surrounding tissue. Thus, inflammation of tissue as usually results from necrotic death is avoided. In vitro, in the absence of phagocytes, the apoptotic cell will eventually enter a degenerative phase called secondary necrosis.
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