CYCLIC NUCLEOLAR CHANGES DURING CELL CYCLE .I. VARIATIONS IN NUMBER SIZE MORPHOLOGY AND POSITION

1. 1. Development of a technique for selective harvesting of synchronous strain L cells has made possible the study of nucleolar changes during the cell cycle. The pattern of DNA synthesis was determined autoradiographically, and cultures which had been planted for 3, 14 and 19 h were chosen as representative of the G1, S and G2 periods, respectively. Changes in morphology, position, number and size of nucleoli were assessed, both cinematographically and using fixed preparations. 2. 2. The significant reduction in nucleolar number between G1 and mitosis was found to be due largely to fusion. Increase in size resulted from both fusion and synthetic activity within the nucleolus. 3. 3. Irregular morphology of nucleoli, marked association with the nuclear membrane and amoeboid movement were characteristic of nucleoli both in G1 and G2. During S phase nucleoli were oval-shaped and centrallylocated; they exhibited much less movement. 4. 4. The possible significance of these cyclic nucleolar phenomena as related to other nuclear changes during the cell cycle has been discussed. © 1968.