Using TUNEL in Combination with an Active Caspase-3 Immunoassay to Identify Cells Undergoing Apoptosis in Preimplantation Mammalian Embryos

The term apoptosis was coined in a now classic paper by Kerr et al. in 1972 (1 ), which summarized the morphological features observed in electron micrographs of cells dying by a process that had been previously termed shrinkage necrosis . It was noted that these features, such as cell shrinkage, nuclear condensation, and fragmentation, along with cellular blebbing and fragmentation, were seen in various tissues and under physiologically normal circumstances. Some features, such as the maintenance of intact organelles, an apparent healthy cytoplasmic membrane, lack of localized inflammation, and frequent phagocytosis of the cell fragments by neighboring cells, suggested that this phenomenon is a normal part of cell turnover and unassociated with large-scale cell damage. The common features of this phenomenon suggested that it was a process under genetic control. The recognition of cells undergoing removal from tissues during normal mammalian embryo development, and cells known to be deleted during the normal development of the nematode Caenorhaditis elegans has confirmed this hypothesis (2 ).