Detection and electron microscopic study of thick cross-striated linear fibrils in mammalian cell nuclei

Using an electron microscope, thick (30-100 nm wide), linear (not branched), cross-striated protein fibrils with an axial repeat of about 65 nm were detected in mammalian cell nuclei. These fibrils differ from the thin filaments of the nuclear matrix described in the literature. Therefore, in this work, the main efforts were aimed at demonstrating the nuclear origin of thick fibrils. Their presence in the material of nuclei destroyed by ultrasound, their contact with isolated nucleoli, and their presence in residual nuclei (nuclear matrix) are shown. Contacts of thick fibrils with both chromatin and the network of filaments of the nuclear matrix were observed. Thick fibrils, which are axial components of condensed chromosomes, are preserved during mitosis. It is likely that their contacts with chromatin and elements of the nuclear matrix are also preserved, ensuring the reproduction of the internal structure of the nuclei in daughter cells. Thick fibrils disintegrate in a medium with low ionic strength. Perhaps this is the reason for their absence in other authors' nuclear matrix preparations. In this work, the nuclei were isolated, and all experiments were carried out in a "complete medium" simulating the intranuclear salt content.