Chromatin structures in ring-shaped nucleoli of human lymphocytes

Ring-shaped nucleoli of mature lymphocytes isolated from the peripheral blood of leukemic and nonleukemic patients were studied with ultrastructural cytochemical methods to determine the presence and organization of chromatin structures in these nucleoli. The chromatin structures are apparently present in "ring-shaped" nucleoli in a number of physical states: (1) as dispersed fine filaments about 20 Å wide, (2) as filaments organized into trabecular structures and (3) as large clusters composed of fibrils 80-120 Å wide. These fibrils were also composed of coiled fine filaments 20 Å in width. The chromatin structures penetrating from the nucleolus associated chromatin were either solitary fine filaments or strips composed of fibrils of filaments. In some sections of ring-shaped nucleoli the peripheral shell was discontinuous and in this region, chromatin structures penetrated into the nucleoli. The central light areas of ring-shaped nucleoli, contained protein as well as chromatin filaments and clusters of fibrils. These findings show that DNA is present in the ring-shaped nucleoli of mature human lymphocytes. Since autoradiographic studies have shown a reduction or inhibition of RNA synthesis in these cells [3, 4, 16] in the course of their maturation, this effect represents a repression of activity of nucleolar DNA rather than its loss from the nucleolus.