Long duration of mitosis and consequences for the cell cycle concept, as seen in the isthmal cells of the mouse pyloric antrum. I. Identification of early and late steps of mitosis

. In this series of two articles, the duration of mitosis and that of the cell cycle were examined in a group of proliferating cells located in the mouse pyloric antrum and known as isthmal cells. However, before measuring the duration of mitosis, as described in the second article, it is necessary to identify the early and late steps of the mitotic process. This is attempted in the present article, in which the four phases of mitosis and the interphase are described in semithin (0.5 μ m thick) Epon serial sections stained with hemalun. The frequency of these phases is then estimated. The beginning of prophase is indicated by the appearance in the nucleus of numerous 0.2‐0.3 μ m thick basophilic threads. The threads gradually increase in thickness to become the typical chromosomes (about 0.7‐ μ m thick) observed at the end of prophase. Metaphase and anaphase show no remarkable features. At telophase, chromosomes separate from one another, gradually acquire pale segments along their length eventually to look like rows of alternating dark and light patches, and finally vanish. When prophases and telophases are defined in this manner, the enumeration of isthmal cells yields a high proportion of prophases (28%) and telophases (31%), but a low proportion of metaphases (1%) and anaphases (0.3%). Forty per cent of the cells are in interphase.