Effects of 6‐azauridine on the embryonic cells of Xenopus laevis, with special reference to nucleolar ultrastructure

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Abstract
Effects of 6‐azauridine on isolated embryonic cells of Xenopus laevis were examined electronmicroscopically. It has been revealed that shortly after the treatment, when the rate of RNA synthesis became reduced, a unique ultrastrutural change occurred in the nucleoli: granular component greatly decreased in amount, while fibrous component retained a rather normal, though somewhat coalesced, net‐work structure. Four to six hours later, when inhibition in the syntheses of protein and DNA became apparent, the nuclear chromatin condensed and the nucleoli became a small round mass which consisted of fibrous component alone in some cells. Finally at the tenth hour, most cells showed pycnotic feature and the nucleoli were fragmented and frequently disappeared. At this time, endoplasmic reticulum and mitochondria were also deformed and decreased in number. The significance of the nucleolar alteration observed has been discussed.