Long duration of mitosis and consequences for the cell cycle concept, as seen in the isthmal cells of the mouse pyloric antrum. II. Duration of mitotic phases and cycle stages, and their relation to one another

. The kinetics of isthmal cells in mouse antrum were examined in three ways: (a) the duration of cell cycle and DNA‐synthesizing (S) stage was measured by the ‘fraction of labelled mitoses’ method; (b) the duration of interphase and mitotic phases was determined from how frequently they occurred; and (c) mice were killed at various intervals after an intravenous injection of 3 H‐thymidine to time the acquisition of label by the various phases of mitosis. The duration of the isthmal cell cycle was found to be 13.8 hr and that of the DNA‐synthesizing (S) stage, 5.8 h. Estimates for the duration of the G 1 and G 2 stages were 6.8 and 1.0 hr, respectively. From the frequency of mitotic phases, defined as indicated in the preceding article (El‐Alfy & Leblond, 1987) and corrected for the probability of their occurence, it was estimated that prophase lasted 4.8 hr; metaphase, 0.2 hr; anaphase, 0.06 hr and telophase, 3.3 hr, while the interphase lasted 5.4 hr. In accordance with this, the duration of the whole mitotic process was 8.4 hr. Ten minutes after an intravenous injection of 3 H‐thymidine, 38% of labelled isthmal cells were in interphase and 62% in early or mid prophase, while cells in late prophase and other mitotic phases were unlabelled. After 60 min, label was in late prophase, after 120 min, in mid telophase and after 180 min, in late telophase. We conclude that there is overlap between some mitotic phases and cycle stages. Thus, while nuclei are at interphase during the early third of S, they are in prophase during the late two‐thirds as well as during G 2 . Also, nuclei are in telophase during the early half of G 1 but at interphase during the late half. Differences in nuclear diameter show that subdivision of both S and G 1 into early and late periods is practical.