Autoradiographic Analysis of Cell Proliferation in Spontaneous Breast Cancer of C3H Mouse. I. Typical Cell Cycle and Timing of DNA Synthesis2

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Abstract
Primary breast tumors of the C3H mouse and autotransplants of these tumors have been labeled in situ by intraperitoneal or intravenous injection of tritiated thymidine. From the frequency of labeled mitotic cells at various times after injection a typical cell cycle has been formulated. This cycle is completed in 1 to 3½ days. The synthesis of deoxyribonucleic acid (DNA) occurs in 9 to 13 hours, and there is a 1- to 4-hour period between the completion of DNA synthesis and the onset of mitosis. In many tumors this typical cycle applies to nearly all dividing cells. A comparison between different tumors and between these tumors and other cell systems indicates that variation in intermitotic time is primarily related to the duration of the presynthetic period. The growth rate of these tumors is limited by factors other than the rate at which the cells synthesize DNA.