Delayed internucleosomal DNA fragmentation in programmed cell death.

DNA fragmentation was evaluated in three instances of programmed cell death, interdigital cell death in embryonic mouse limbs, and metamorphic death of both the labial glands and intersegmental muscle in the tobacco hornworm Manduca sexta. In the mouse, we evaluated both developmental cell death and expanded-range cell death induced by retinoic acid. The status of DNA was examined in several ways. Nuclei were examined by electron microscopy and Feulgen staining. Quantitative assessment of total DNA content in Feulgen-stained degenerating nuclei was made for the gland. In the labial gland, DNA content does not drop during the early phases of cell death; nor is an endonucleolytic ladder seen when DNA was examined by ethidium bromide staining or prelabeling with [3H]thymidine. Only by using end labeling of DNA could we detect DNA fragmentation at a very late stage in cell death, day 4 of the collapse of the gland. In contrast, WEHI 7.1 lymphoma cells display an early and extensive ladder after treatment with glucocorticoids. In mouse limb, for which cell death follows a more classic apoptotic morphology, a ladder is likewise not seen. We conclude that activation of an endonuclease is neither a trigger nor a necessary or defining component of the early phases of developmental programmed cell death, and that reported failure by others to find such a ladder may depend on limitations in the system that is under investigation.