Nucleolar activity in differentiated cells after stimulation.

Initiation of nucleolar organizer region (NOR) activity was observed by using the silver staining method at various times after activation or stimulation of differentiated cells. Two methods were used: (1) activation of human lymphocytes by treatment with phytohemagglutinin (PHA), and (2) cell-cell fusion of chick erythrocytes with squirrel monkey cells. An increase in NOR activity in lymphocytes was seen as early as 4 h after PHA treatment and between 10 and 22 h in the chick erythrocytes after fusion. In both systems, as the size of the dormant cell nucleus increased, the amount of silver staining increased until the silver-stained area approached that of cycling cells.