Anchorage of the nucleolus in the pore complex-lamina by a DNA-bearing structure masked in situ in rat liver nuclei.

We have developed a method by which nuclear shells containing nucleoli can be isolated from membrane-depleted rat liver nuclei. This method involves the removal of the internal chromatin. This chromatin is expelled from the nuclear shell using combinations of low and high ionic strength buffers. The expelled internal part is subsequently digested with DNase I or micrococcal nuclease. Examination by electron microscopy of the nuclear and the nucleolar structures at various steps of the isolation procedure shows that the nucleoli are anchored in the peripheral lamina by a pedicle that is continuous with an intranucleolar network. This network is masked in situ by nucleolar granules. The pedicle and the network which support the nucleolar DNA are composed mainly of non-histone proteins insoluble in 2M NaCl.