We have used high resolution scanning electron microscopy (SEM) to study the nuclear envelope components of isolated mouse liver nuclei. The surfaces of intact nuclei are covered by closely packed ribosomes which are distinguishable by SEM from nuclear pore complexes. After removal of nuclear membranes with the nonionic detergent Triton X-100, the pore complexes remain attached to an underlying, peripheral nuclear lamina, as described by others. The surface of this dense lamina is composed of particulate granules, 75-150 A in diameter, which are contiguous over the entire periphery. We did not observe the pore-topore fibril network suggested by other investigators, but such a structure might be the framework upon which the dense lamina is formed. Morphometric analysis of pores and pore complexes shows their size, structure, and density to be similar to that of other mammalian cells. In addition, several types of pore complexassociated structures, not previously reported by other electron microscope (EM) techniques, are observed by SEM. Our studies suggest that the major role of the dense lamina is associated with the distribution, stability, and perhaps, biogenesis of nuclear pore complexes. Treatment of isolated nuclei with a combination of Triton X-100 and sodium deoxycholate removes membranes, dense lamina, and nuclear pore complexes. The resulting "chromatin nuclei" retain their integrity despite the absence of any limiting peripheral structures.Despite the attention devoted to the nuclear envelope, our knowledge of the organization, function, and biogenesis of this complex membrane system is still incomplete. There is sufficient experimental evidence to suggest its role in nuclear-cytoplasmic exchange (39,40,45) and in the organization of interphase chromatin (see references 12, 14, 17, 25, 44, and 50 for recent reviews) but the mechanisms involved are poorly understood. In part, this is due to a lack of certainty about the structure and interrelationships of the various envelope components to each other and to the underlying chromatin and associated nucleoplasmic substances. For example, current methods of exploring the morphological relationship of nuclear pores to outer membrane ribosomes require extrapolation of data from either multiple thin sections (13, 24) or from freeze-etched sections (49) to reconstruct surface topography. Unfortunately, freeze-cleaving produces uncertain fracture planes, and heavy 118
