A key feature of apoptosis is that cells undergoing this programmed form of death are recognized by phagocytes and irgested while still intact, protecting tizsues from the potentially harmful consequences of exposure to the contents of the dying cells. This article reviews recent data whtch indicate that phagocyte recognition of apoptotic cells as' senescent-self'involves at least three classes of receptors on the phagocyte surface, while apoptotic cells may display their'edible'status in a number of different ways.

The purpose of this review is to examine current knowledge of the mechanisms by which phagocytes recognize and ingest cells undergoing apoptosis. This is a physiological or'programmed'form of cell death, distinct from'accidental'cell death or necrosis, and is responsible for deletion of unwanted cells in processes as diverse as immunological tolerance, embryological remodelling, neoplasia, inflammation and normal tissue turnover/" 4. Most readers will be familiar with the morphologica! characteristics of cells undergoing apoptosis, such ss condensation ot nuclear heterochromatin or fragmentation into membrane-bound apoptotic bodies, which accompany biochemical changes such as internucleosomal cleavage of chromatin into low molecular weight fragments 5. As described in the preceding article by John Cohen, the mechanisms which determine and regulate apoptosis have received much recent attention. However, less interest has been shown in mechanisms underlying the usual physiological fate of cells undergoing apoptosis in vivo-swift recognition and ingestion of the intact cell by local phagocytes before final destruction by the lysosomal enzymes of the phagocyte, all without causing damage to neighbouring cells or eliciting an inflammatory response z'3. This last point is beautifully illustrated in the development of the Drosophila eye, where to achieve the adult form, thousands of unwanted interommatidal cells undergo programmed death and phagocytosis without disrupting the delicate architecture of the organ 6.