PROBING NUCLEAR ULTRASTRUCTURE BY ELECTRON SPECTROSCOPIC IMAGING

Mammalian nuclei are complex organelles containing many functionally d istinct nucleoprotein and protein particles in the size range 20-30 nm . This complexity hinders the study of structure-function relationship s within the mammalian nucleus. Element-specific mapping using the ene rgy-filtered transmission electron microscope can provide novel inform ation on protein and nucleic acid density within structures, facilitat ing the identification of biochemical heterogeneity within morphologic ally similar structures. We demonstrate that imaging phosphorus, nitro gen and carbon can be useful in the characterization of protein and nu cleoprotein structures within the nucleus. Additionally, electron spec troscopic imaging (ESI) may be used to map the distribution of stains relative to unstained material when biochemical-specific staining prot ocols, such as EDTA-regressive staining of RNA with uranyl acetate, ar e used. Relative mass may also be determined from ESI images and can b e combined with elemental information further to distinguish biologica l constituents. Using this approach, heterochromatin was found to be v ariable in nucleic acid content although the morphology appeared relat ively homogeneous. ESI shows substantial promise for the investigation of structure-function relationships in biological specimens.