Electron Microscopic Radioautography: The Sites of Nuclear RNA Synthesis in Early Embryonic Cells
The relationship between RNA synthesis and ultrastructural organization of chromatin fibrils was examined in the interphase nuclei of the sea urchin embryos (Arbacia punctulata). The embryos at various stages of development were treated for 1 or 3 hours in sea water with 50 μC/ml of 3H-5-uridine. The samples were fixed in OSO4 or glutaraldehyde (GTA) and embedded in Epon or glycol methacrylate (GMA). Radioautographs were made by applying a thin film of Ilford L4 emulsion directly from an expandable wire loop to thin sections mounted on grids. Following exposure of 10 to 20 weeks, they were developed in Microdol-X or physical developer.In the cleavage and early blastula embryos, each interphase nucleus contains diffusely distributed chromatin and shows only very low incorporation of 3H-uridine into RNA (Fig.1). During these periods, many nucleolus-like bodies occur within the nucleus although they are not labeled with the isotope.