Nuclear surface complex as observed with the high resolution scanning electron microscope. Visualization of the membrane surfaces of the neclear envelope and the nuclear cortex from Xenopus laevis oocytes

Abstract

The nuclear envelope and associated structures from X. laevis oocytes (stage VI) were examined with the high resolution scanning electron microscope (SEM). The features of the inner and outer surfaces of the nuclear surface complex were revealed by manual isolation, whereas the membranes facing the perinuclear space (the space between the inner and outer nuclear membranes) were observed by fracturing the nuclear envelope in this plane and splaying the corresponding regions apart. Pore complexes were observed on all 4 membrane surfaces of this double-membraned structure. The densely packed pore complexes (55/.mu.m2) are often clustered into triplets with shared walls (outer diameter = 90 nm; inner diameter = 25 nm; wall thickness = .apprx. 30 nm), and project .apprx. 20 nm above each membrane except where they are flush with the innermost surface. The pore complex appears to be an aggregate of 4 30 nm subunits. The nuclear cortex, a fibrous layer (300 nm thickness) associated with the inner surface of the nuclear envelope, was revealed by rapid fixation. This cortical layer is interrupted by funnel-shaped intranuclear channels (120-640 nm diameter) which narrow towards the pore complexes. Chains of particles, arranged in spirals, are inserted into these intranuclear channels. The fibers associated with the innermost face of the nuclear envelope can be extracted with 0.6 M KI to reveal the pore complexes. A model of the nuclear surface complex, compiled from the visualization of all the membrane faces and the nuclear cortex, demonstrates relations between the intranuclear channels (3.2/.mu.m2) and the numerous pore complexes and the possibility of their role in nucleocytoplasmic interactions.