Structural Aspects of the Nucleolus and its Associated Heterochromatin in Neurons

The structure of the nucleolus associated chromatin in avian neurons is described. It consists of one or two great heterochromatic blocks applied side by side to the nucleolus. Included within the nucleolar mass there are chromatinic filaments originated from the heterochromatic blocks. Analysis of autoradiographical preparations reveals that the nucleolus associated chromatin incorporates 3H uridine and 3H thymidine at a high rate. It is hypothetically postulated for this cellular type, that the process of formation and accumulation of the nucleolar substance in the middle zone between the heterochromatic blocks could be comparable to the formation of “puffs” in dipteran polythene chromosomes.