"Large" and "small" nuclear pore complexes; the influence of glutaraldehyde

Aliquots of lymphocyte cell suspensions were pretreated according to the following three schedules before freeze fracturing: (a) prefixed with 2% glutaraldehyde before infiltration with 25% glycerol in medium RPMI-1640; (b) frozen in medium RPMI-1640 without additional pretreatment; and (c) frozen after pretreatment with 25% glycerol in medium RPMI-1640. The diameters of the fractured nuclear pore complexes of cells prefixed with glutaraldehyde were normally distributed within the range 70-120 nm (median 90 nm). The nuclear envelopes of 66-75% of cells processed through schedules b and c, which omitted glutaraldehyde fixation, had 70-120 nm diameter pores, while the remainder had pores with diameters in the range 120-175 nm. The large pores were structurally similar to the smaller pores except for their dimensions. These results indicate that glutaraldehyde gives rise to shrinkage of the larger pores to the minimum, smaller, diameter. Apparent orifices of at least 30 nm diameter were sometimes observed at the centres of these large pore complexes. We propose that the variation in pore diameters may indicate opening and closure of this orifice, and that the widely reported "central granule" of the nuclear pore complex corresponds with the orifice in a closed configuration.