New aspects of intranuclear structures following partial decondensation of chromatin: a cytochemical and high-resolution autoradiographical study

Abstract

Exposure of cultured CVI monkey kidney cells to a hypotonic medium containing a nonionic detergent induces a partial decondensation of nuclear components which can be studied advantageously in ultrathin sections. In addition to previously described 20–25 nm, DNA-containing chromatin fibres arrayed perpendicularly to the nuclear lamina, a new nuclear configuration was found which consists of abundant clusters of 6–7 nm fibrils. These ‘filamentous masses’ are the most heavily labelled component of the nucleus in autoradiographs after brief (2 min) exposure to [5-3H]uridine, and they are greatly diminished in number after treatment of the cells with a low dose of actinomycin D. Therefore, we interpret the filamentous masses to be nascent RNP and to represent the fibrillar regions of the nucleolus which have become decondensed into partially spread ‘Christmas-tree’-like figures detectable in situ in ultrathin sections. Similar structures occur in all cell types examined to date. This paper outlines the preparative procedure.