Changes in nucleolar structure, number and size in cellular activation and inactivation. Observations in human phytohaemagglutinin-treated lymphocytes.

Lymphocytes in human peripheral blood are small, relatively inactive cells. The addition of phytohaemagglutinin (PHA) to cultures of these cells causes a marked increase in cellular and nuclear size, DNA-synthesis and metabolic activity, which reaches a maximum three days after the onset of culturing. The cells then undergo an inactivation process over a period of approximately ten days by which time they have reverted to cells resembling small, inactive lymphocytes. Within the first three days, nucleoli increase in size and number, changing from ring-shaped to nucleolonema-exhibiting to compact nucleoli. In the course of the inactivation process the nucleoli decrease in size and change from compact nucleoli directly into ring-shaped nucleoli. Thus activation and inactivation pathways are different. There is an increase in the number of nucleoli during the inactivation phase up to the seventh day in culture, followed by a slight decrease until day 14. This suggests that nucleoli in metabolically active cells have a tendency to fuse, whereas those in inactive cells tend to fragment.