Effects of divalent-cation chelators and chloramphenicol on the spatial relationship of the nuclear envelope to chromatin in micronuclei of Chinese hamster cells.

In the presence of the spindle poison Colcemid in the culture medium to prevent anaphase, approximately 20% of Chinese hamster metaphase cell were converted to micronucleated cells during 7 h. In the micronuclei the chromosome had become enclosed by a nuclear envelope (NE). In the light-microscope the micronuclei were of two kinds: with either visible chromatids or with decondensed chromosomes. In the electron microscope (EM) the spatial relationship of the NE to the chromatin was of two kinds only in the presence of Colcemid. In about 90% of the micronucleated cells the spatial relationship was normal, ie, the NE was immediately adjacent to the chromatin. In the remaining cells, the NE was distended so that the outer NE was separated from the inner one. In the presence of the divalent cation chelator, (ethylenedinitrilo) tetraacetic acid (EDTA) or the Ca2+-chelator [ethylenebis (oxyethylenenitrilo)] tetraacetic acid (EGTA) in addition to Colcemid, the amount of cells with micronuclei increased to 40%. The light-microscope appearance was the same as that found in the absence of the chelating agents. However, after Colcemid plus EGTA, EM revealed that only about 50% of the micronucleated cells had NE that was immediately adjacent to the chromatin and about 10% of them had distended outer NE. In the remaining 40% a third kind of spatial relationship was seen: the NE was intact but most of it was not adjacent to the chromatin. Furthermore, this type of micronucleus often contained mitochondria within the confines of NE. Thus, Ca2+ and possibly Mg2+ may regulate the rate of formation of the NE and also its ultrastructural relation to the chromatin. Mitochondrial function also appears to be involved in this relationship. In the presence of chloramphenicol (CAP), an inhibitor of mitochondrial protein synthesis, in addition to Colcemid, only about 50% of the micronucleated cells exhibited the normal relationship. The outer NE was separated from the inner NE in about 46% of the micronucleated cells and the third kind of NE-chromatin relationship was observed only in 2%. In the case of the third kind of relationship produced by CAP, inclusion of mitochondria within the micronuclei was not observed, in contrast to the finding with EGTA.