Inhibition of Nuclear Reconstruction by Dithiothreitol

sequential processes that occur during nuclear reconstitution : first each individual chromosome becomes surrounded by a double membrane with pores characteristic of a nuclear membrane (Harris, 1962; Longo, 1972), then the membranebounded chromosome vesicles (karyomeres) fuse Sea Urchins to form a single membrane-bounded nucleus containing the full complement of chromosomes (Wilson, 1925 ; Mazia, 1961 ; Longo, 1972) . This sequence of events suggests that it should be possible to obtain chromosomes separated during interphase, each in its own "nuclear" membrane, by inhibiting the second step of nuclear reconstitution, the fusion of chromosomal vesicles . Such a cell would then possess many small micronuclei instead of one large nucleus ; in fact, as many micronuclei as there are chromosomes . In an experiment involving the application of dithiothreitol (DTT) to cleaving sea urchin embryos, we noticed a distinct change in the morphology of all interphase nuclei. In each blastomere of twoor four-cell embryos that had been incubated in the presence of DTT from before the first cleavage, the nucleus appeared multivesiculated. It occurred to us that the multivesiculated nucleus might actually be an aggregate of unfused karyomeres . Earlier studies on dividing cells and of the effects of compounds that block the formation of disulfide bonds have noted interferences with the normal dynamic behavior of the nuclear membrane at mitosis (Hughes, 1949; Mazia, 1961) . It therefore seemed reasonable to hypothesize that DTT was producing a multivesiculate nucleus by inDTT BRIEF NOTES