Laval Univ. Cancer Research Centre, Québec, Canada The nuclear lamina is usually thought of as a rigid structure which surrounds the nuclear contents and also projects into the interior forming nuclear channels. Here experiments will be presented which show that the external surface area of the lamina can increase reversibly or decrease. Using nuclei from K562 cells attached to polylysine-coated slides, a reversible increase of the surface area was seen in two conditions. This area increased ≈1.5-fold (≈two-fold increase of nuclear volume) upon transfer from a physiological buffer slightly modified from that of Cook to a buffer containing KCl and Na2HPO4 at 1/100 of their concentration, and the nuclei contracted instantaneously to their normal size upon return to physiological buffer. Second, nuclei expanded to about twice their volume when microinjected with physiological buffer (containing fluorescent 77 kDa dextran to verify the absence of leakage) and they contracted instantaneously to their normal size when the injection pressure was released. Inversely, when the concentration of macromolecules in nuclei was reduced by incubation with restriction enzymes and electroelution of the excised chromatin fragments, the nuclei contracted as more chromatin was removed, implying that the nuclear envelope exerted pressure on the remaining nuclear contents. All these responses are due to properties of the nuclear lamina, since they were seen after membranous components of the envelope were extracted with Triton X-100. Immunolabelling studies will be shown to distinguish between two mechanisms for these changes of lamina area: elasticity and stretching of the lamina itself, or externalisation/internalisation of lamina material in nuclear channels. These experiments suggest that the nuclear lamina is a quite flexible elastic structure which normally exerts pressure on the intranuclear macromolecules.
