O verview
Cell death can be classified according to m orphological, biochemical or circumstantial criteria (W yllie et al., 1980). Programmed cell death, or apoptosis, has been show n in recent years to play a key role in m orphogenesis during the development o f many organ system s, where it is used by the embryo as a tool to eliminate unwanted cells. The products o f programmed cell death, apoptotic bodies, are rapidly phagocytosed and cleared, so that few dead cells are found in the system at any one time (Raff, 1992). It is perhaps because o f this rapid clearance that the extent and importance o f developmental cell death has been largely overlooked until recent years.
The extent and time-course o f programmed cell death that occurs during development differs between each organ system. The apoptosis found in the developing limb during the remodelling o f the footplate giving distinct separate digits is “castastrophic”, occurring over a relatively short period o f time, and resulting in a dramatic change o f shape. The programmed cell death that occurs during the developm ent o f the kidney is a more gradual process, described as “trickle-like”, resulting in no gross morphological change in kidney shape.
It is still unclear why certain cells die and how the programme o f cell death is regulated. There are many different signalling pathways, intracellular and extracellular, which ultimately activate a comm on cell death programme. H ow ever, it is agreed that aU nucleated cells constitutively express the protein components required for cell death. A proteolytic cascade is involved in the execution o f this pathway. A fam ily o f specific intracellular regulatory proteins, caspases (the “c ” refers to the cysteine protease activity, w hile the “aspase” refers to the cleavage after aspartic acid; Alnemri et al., 1996), are
involved in its execution, and during development it may also be controlled at the transcriptional level (Jacobson et al., 1997). The cell death pathway has been found to be similar between tissues and across species barriers, and is even conserved between invertebrates and vertebrates. This has enabled researchers to extrapolate from the genetic “cell death” studies in the worm C.elegans, and discover genes dedicated to programmed cell death in vertebrates by virtue o f their hom ology to the c ed genes.
A p o p to sis differs f r o m necrosis in a num ber o f w a ys
A poptosis is an active “programmed” process o f gene-directed cellular destruction serving a biologically significant homeostatic function. N ecrosis is a degenerative form o f death which is usually the result o f som e environmental occurrence which is accidental. Generally the two death types can be separated according to the follow ing criteria shown below (Thompson e t a l , 1992).
Apoptosis
Necrosis
chromatin condensation chromatin degeneration
cell shrinkage cellular swelling
plasm a membrane & organelles intact plasma & organelle membranes mpture
under control o f physiological stimuli result o f injury (toxins orischemia)
affects single cells affects group o f cells
The first morphological sign o f apoptosis is compaction o f the nuclear chromatin and the condensation o f the cytoplasm. This condensation is accompanied by convolution o f the nuclear and cellular membranes and the nucleus breaks into smaller fragments. Membrane-bound spherical or ovoid apoptotic bodies are produced. The internucleosomal D N A is cleaved by endonucleases into multiple fragments o f 185-200 base pairs, which gives the classic D N A laddering effect if run on a gel (Arends et al.,
1990).
The genetics o f the death program m e are unfolding fro m studies in C.elegans and Drosophila
Programmed cell death has been extensively studied in the worm C.elegans, giving a clear model for understanding the basic genetic principles o f developmental apoptosis. During the developm ent o f C.elegans, 1090 cells are formed, o f w hich 131 undergo cell death (Sulston and Horvitz, 1977; Sulston et al., 1983). These 131 cells die at a particular time, in a precise manner, in every worm (Ellis et a i , 1991b). The dying cells show the characteristic signs o f apoptosis; the cell as a w hole rounds up, the nuclear chromatin condenses and the eytoplasm contracts (Ellis et al., 1991b). These dead cells are rapidly engulfed and degraded by neighbouring cells (Ellis et a i , 1991a). The deaths occur even in the absence o f neighbouring cells, indicating that they are pre-programmed to die and not killed; the death is “suicide”, rather than “murder” (Miura and Yuan,
1996).
Genetically four steps o f programmed cell death can be identified; suppression, execution, engulfm ent and degradation. Ceds and more recently cell-death specification
(ces) genes have been identified in C.elegans as key players in somatic cell deaths (F ig. 1.2; H edgecock e t al., 1983; E llis and Horvitz, 1991).