• nuclear matrix;
  • filaments;
  • lymphoma;
  • nucleus

While much evidence indicates a high degree of spatial organization in the nucleus, the underlying molecular structures that support it remain poorly characterized. By extracting with high concentrations of RNase A in a modification of the sequential extraction protocol of Penman, we have identified a novel intranuclear network in the mouse lymphoma cell line, EL-4. Micrographs of embedment-free sections of extracted cells reveal anastomosing filaments of two different diameters: 3–5nm and 8–10nm. The 3–5-nm filaments are interconnected in many junctions and appear to blend smoothly into each other. The 8–10-nm fibers frequently split into two 3–5-nm filaments. Some 3–5-nm fibers appear to be connected at 90° angles with the 8–10-nm fibers. All junctions are smooth with no apparent junction protein. Flow cytometric analysis of RNase A- (and DNase I-) extracted nuclear matrices indicates that they do not contain significant amounts of protein that react with anti-actin and anti-vimentin monoclonal antibodies. Extraction of EL-4 nuclear matrices with high salt does not reveal 8–10-nm core filaments described after similar treatment of tumor cell lines of cervical and mammary origin. The novel characteristics of the core filaments in EL-4 lymphoma cells may reflect cell-type specificity of the nuclear matrix.