Crisis periods and apoptotic commitment: Death imprints?

Crisis periods in development are critical periods of cell death that have long been suggested as "epigenetic crises" which are central to normal and abnormal embryogenesis. Under in-vitro culture conditions, there are simila r crisis periods or Hayflick limits of culture senescence. Epigenetic modul ations from CpG methylation coupled to DNA replication provide an alternate timing mechanism to the telomere/telomerase biological clock. Physiologica l cell death in both development and in in-vitro isolates is primarily apop totic. Arguments of divergent apoptotic death commitments as caspase-depend ent and -independent pathways seem to suggest that there is no possibility of a global life-and-death signal. However recent reports implicating CpG-s pecific cleavage in apoptosis implies that the powerful imprint mark that s ilences genes, protects genes from nuclease restriction, and modulates chro matin conformations, could provide a common commitment pathway of convergen ce in the death cascade. If the imprint mark were central to the apoptotic commitment, then apoptosis is Lamarckian not Darwinian.