Direct visualization of a protein nuclear architecture

Whether the cell nucleus is organized by an underlying architecture analago us to the cytoskeleton has been a highly contentious issue since the origin al isolation of a nuclease and salt-resistant nuclear matrix. Despite elect ron microscopy studies that show that a nuclear architecture can be visuali zed after fractionation, the necessity to elute chromatin to visualize this structure has hindered general acceptance of a karyoskeleton. Using an ana lytical electron microscopy method capable of quantitative elemental analys is, electron spectroscopic imaging, we show that the majority of the fine s tructure within interchromatin regions of the cell nucleus in fixed whole c ells is not nucleoprotein. Rather, this fine structure is compositionally s imilar to known protein-based cellular structures of the cytoplasm. This st udy is the first demonstration of a protein network in unfractionated and u ninfected cells and provides a method for the ultrastructural characterizat ion of the interaction of this protein architecture with chromatin and ribo nucleoprotein elements of the cell nucleus.