Active participation of cell death in development and organismal homeostasis. 
The precise, regional execution of programmed cell death is required for the proper patterning and sculpting of the embryonic primordium during animal development.  In addition, cell death that is not directly involved in sculpting is also widely observed.  The most abundant morphological form of programmed cell death in developing animals is apoptosis, and identification of the apoptotic genetic pathways has enabled the study of apoptosis' regulation and roles during development.  Genetic and bio-imaging studies have permitted the study of the active roles of cell death in development and organismal homeostasis.