The fine structure of the nuclei of Tetrahymena pyriformis throughout the cell cycle.

The fine structure of the nuclei of logarithmically growing Tetrahymena pyriformis, strain HSM, was studied at 30-minute intervals throughout the cell cycle. Organisms were selected at similar stages of cytokinesis by means of a braking pipette, incubated, fixed in OsO(4), and embedded in agar to facilitate subsequent preparation for electron microscopy. Aggregates of micronuclear chromatin underwent a decrease in density and number with a concomitant increase in size throughout interphase. There were no impressive changes in macronuclear morphology. It was found possible to estimate a cell's progress through interphase by observation of micronuclear morphology, but attempts to correlate changes in fine structure with periods of DNA synthesis were unsuccessful.