Nucleolar development in the interphase of the cell cycle.

Stereology of nucleoli at 3 different points of the cell cycle, in the middle of the Gu S and Gt of interphase, was accomplished in a naturally synchronous cell population rendered binucleate and thus 'labelled' and made identifiable by 1 h caffeine treatment in root meristems of AUium cepa L. Consistent structural changes were found so that nucleolar parameters by themselves can locate a cell at any interphase period. The growth of the nucleolus in the course of interphase takes place exclusively in its granular portion. The growth rate of this last component was found to be greater in the first half of interphase (5-4/tm' h") than in the second (29 /im h"), as calculated for a nucleus with only one fused nucleolus. The changes in volume of the nucleolar components during interphase do not parallel the gene dosage. Nucleolar surface at each point seems to be the factor which determines the growth rate in the preceding interval, since these rates are inverted for fused and unfused nucleoli. The nucleolar volume occupied by lacunae is minimal in the »S-period and shows an enormous increase by the middle of Ga. We think our data may be the structural basis on which a model for nucleolar functioning in proliferating cells could be built.