Changes in nucleolar number of ascites tumor cells treated with actinomycin D.

In the nuclei of Ehrligh and NK/Ly mouse ascites tumor cells the number of nucleoli varies between 1 and 10. The average nucleolar number is 3.3 +/- 1.3 for both tumors. This value as well as the distribution of nuclei with various nucleolar number are parctically constant between the 5th and 15th day of transplantation. The nucleolar changes resulting from the effect of a single 200 mug/kg i.p. dose of actinomycin D take place in two phases, almost in the same manner for both tumors. Within 24 hours the mean nucleolar number decreases by about 20 percent accompanied by a diminution of size of nucleoli. 15 to 20 percent of nuclei does not contain real nucleoli but has small spots showing nucleolus-like staining ("scattered" nucleolar cells). The second phase is characterized by a 25-35 percent increase in the mean nucleolar number followed by a nucleolar enlargement. The process returns to normal within 6-7 days.