INTERPHASE NUCLEI OF MANY MAMMALIAN-CELL TYPES CONTAIN DEEP, DYNAMIC,TUBULAR MEMBRANE-BOUND INVAGINATIONS OF THE NUCLEAR-ENVELOPE

The nuclear envelope consists of a double-membraned extension of the r ough endoplasmic reticulum. In this report we describe long, dynamic t ubular channels, derived from the nuclear envelope, that extend deep i nto the nucleoplasm. These channels show cell-type specific morphologi es ranging from single short stubs to multiple, complex branched struc tures. Some channels transect the nucleus entirely, opening at two sep arate points on the nuclear surface, while others terminate at or clos e to nucleoli. These channels are distinct from other topological feat ures of the nuclear envelope, such as lobes or folds. The channel wall consists of two membranes continuous with the nuclear envelope, studd ed with features indistinguishable from nuclear pore complexes, and de corated on the nucleoplasmic surface with lamins. The enclosed core is continuous with the cytoplasm, and the lumenal space between the memb ranes contains soluble ER-resident proteins (protein disulphide isomer ase and glucose-6-phosphatase). Nuclear channels are also found in liv e cells labeled with the lipophilic dye DiOC(6). Time-lapse imaging of DiOC(6)-labeled cells shows that the channels undergo changes in morp hology and spatial distribution within the interphase nucleus on a tim escale of minutes. The presence of a cytoplasmic core and nuclear pore complexes in the channel walls suggests a possible role for these str uctures in nucleo-cytoplasmic transport. The clear association of a su bset of these structures with nucleoli would also be consistent with s uch a transport role.