Transcriptionally Active Amphibian Oocyte Nucleolar Chromatin is Organized in Higher Order Structure

Abstract

The structure of transcriptionally active chromatin of amplified oocyte nucleoli of amphibians has been studied in detail (Miller, Beatty, 1969; Scheer, Zentgraf, 1982). It has been shown that tandem arrays of transcription units (TUs) are the basic structural elements transcribed in chromatin of nucleoli. However, light microscope data on the structure of nucleoli during oogenesis give some grounds to assume that transcriptionally active chromatin of nucleoli is organized into a higher order structure, as compared to the linearly arranged TUs. For instance, in vitellogenic amphibian oocytes, the spherical nucleoli acquire the appearance of bead-string rings (Callan, 1966). Ring structures with beads along the axis have been found directly in the body of spherical nucleoli during examination of thin section oocyte nuclei (Parfenov, 1983). The question of the correspondence of structures observed by different methods of the investigation is still to be answered. The goal of this study was to examine the amplified nucleoli by light and electron microscopy and to describe the organization of their transcriptionally active chromatin.