In the absence of a clearly defined biochemical event that can be considered as the point-ofno-
return, the NCCD proposes that a cell should be considered dead when any one of the
following molecular or morphological criteria is met: (1) the cell has lost the integrity of its
plasma membrane, as defined by the incorporation of vital dyes (e.g., PI) in vitro; (2) the cell,
including its nucleus, has undergone complete fragmentation into discrete bodies (which are
frequently referred to as ‘apoptotic bodies’); and/or (3) its corpse (or its fragments) has been
engulfed by an adjacent cell in vivo. Thus, bona fide ‘dead cells’ would be different from ‘dying
cells’ that have not yet concluded their demise (which can occur through a variety of
biochemically distinct pathways, see below). In particular, cells that are arrested in the cell
cycle (as it occurs during senescence) should be considered as alive, and the expression
‘replicative cell death’ (which alludes to the loss of clonogenic potential), as it is frequently
used by radiobiologists, should be abandoned.