MULTIPLE PATHWAYS FOR APOPTOTIC NUCLEAR FRAGMENTATION

We analyzed the ultrastructure of apoptotic nuclear fragmentation in U 937 cells treated with many different apoptogenic agents. We found tha t this characteristic apoptotic feature can be achieved through multip le alternative pathways, depending on the apoptogenic inducer, leading to slightly different final nuclear morphologies. In most instances, the irregularly shaped nucleus of U937 rounds up; then, chromatin cond enses at the nuclear periphery, Condensed chromatin can form protrudin g patches, which eventually bud from the nucleus in sealed vesicles th rough a process which is actin-dependent, since it could be blocked by cytochalasins. Alternatively, chromatin condenses in tiny, nonprotrud ing crescents, and a cleavage in the nuclear sap forms, beginning from the inner nuclear membrane and growing inward, thus splitting the nuc leus. In U937 induced to apoptosis by hydrogen peroxide in the presenc e of ADP-ribosylation inhibitors, the nuclei fragment in many vesicles before chromatin even beans to condense: chromatin condensation proba bly occurs as a consequence. While all the apoptotic morphologies desc ribed above evolve from interphase cells, a peculiar apoptotic morphol ogy, possibly deriving from mitotic cells, is detected upon oxidative stress, recalling the formation of micronuclei by clastogenic treatmen ts; it shows partially membrane-bound chromatin patches, which look mi dway between condensed chromosomes and apoptotic condensed chromatin, The existence of these multiple pathways for nuclear fragmentation may indicate an evolutionary convergence, suggesting that this event may play an important physiological role in apoptosis. (C) 1996 Academic P ress, Inc.