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Formation of Microspherules in Nucleoli of Tumor Cells Treated with High Doses of Actinomycin D¹

Department of Pharmacology, Baylor University College of Medicine, Houston, Texas 77025

Microspherules, composed of condensed and segregated fibrillar components, were observed in the nucleoli of the Ehrlich ascites cells and in those of the Novikoff hepatoma cells after treatment with high doses of actinomycin D, but not in Walker tumor cells. The number of these microspherules was calculated to range from 100 to 300 per nucleolus. The density of the microspherules decreased markedly after treatment with pepsin and ribonuclease, but not after treatment with deoxyribonuclease. The microspherules contain both protein and RNA and are not aggregations of chromatin as evidenced by their lack of DNA. In the Walker tumor cells, compact nucleolonemas, segregation, and a decreased amount of granular components and vacuole-like areas were observed in the nucleoli after treatment with actinomycin D.

¹ Supported by grants from the American Cancer Society, the USPHS (CA-8182), and the Jane Coffin Childs Fund.

Received 7/ 7/66. Accepted 3/ 7/67.