NUCLEAR RECOVERY FROM CENTRIFUGATION-CAUSED ELONGATION - INVOLVEMENT OF THE MICROFILAMENT SYSTEM IN THE NUCLEAR PLASTICITY

The plasticity of elongated nuclei with thread-like basal protrusions was investigated after centrifuging protonemal cells of the fern Adian tum capillus-veneris basipetally for 2 or 3 hr. The morphological reco very of the nuclei including the shortening process of the thread coul d directly be visualized by video microscopy of nuclei with bubble-lik e thread ends in centrifuged, living cells. The shortening proceeded i n three phases: (1) the fast shortening of the part between the bubble and the nuclear apical main body (NAMB),(2)the slow shrinking of the bubble,(3)the entrance of the nucleolus into the NAME. Although the th read shortening process was quite uniform, there were irregularities l ike reextension of the threads over short distances, The experimental system of elongated nuclei was used to probe the role of the cytoskele ton in the nuclear plasticity. Directly after basipetal centrifugation , thick strands of microfilaments (MFs) were found to be aligned with the nuclear threads, whereas microtubules (MTs) were not in cytoskelet on-depolymerizing experiments, cytochalasin B caused a reduction of th e shortening process, showing that the MF system in the cytoplasm is i nvolved in the nuclear recovery, In non-centrifuged as well as in cent rifuged cells, on the other hand, cytoplasmic streaming was efficientl y inhibited by cytochalasin B, whereas it was not significantly affect ed by colchicine, The moderate effect of cytochalasin B on the thread shortening suggests that still other driving forces such as tension in the nuclear envelope and perhaps intranuclear forces are involved in the thread shortening mechanism.