Rapid cell death of mouse intestinal crypt cells was investigated after 1000 rads of x-irradiation by counting pyknotic cells in tissue sections. The number of normal crypt cells decreased with the appearance of pyknotic cells after irradiation. Pyknotic cells appeared within 30 min and the number of pyknotic cells reached a maximum 4 hr after irradiation. From the disappearance rate of the cells the mean life span of pyknotic cells was estimated to be 2 hr or less. Pyknotic cells observed within 4 hr after irradiation were produced without cell division, and this interphase death by pyknosis occurred in all phases of cell cycle. The tendency for the cells to develop into pyknotic cells was high in G2 phase, intermediate in S phase, low in G1 phase. The increase in mitotic index and the decrease in pyknotic cells from the second to the fifth day after irradiation were inversely related. After the disappearance of pyknotic cells, rapid recovery of the cell population took place with logarithmic cell proliferation and shortening of the cell cycle. Recovery to the normal cell population level was attained within 5 days after irradiation.

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