Mitosis in Mucor hiemalis has been studied during life, in fixed and stained preparations and in electron micrographs of sections. The chromatin occupies the periphery of resting as well as dividing nuclei. It could not be resolved into separately visible chromosomes and was never seen arranged in the form of a metaphase plate. Division of the nucleus and of the nucleolus within is achieved by elongation followed by constriction. The unvarying optical contrast between nucleoplasm and cytoplasm during division implies that mitosis takes place within the intact nuclear envelope.
Electron microscopy reveals that division is accompanied by the development of a short, straight, slender bundle of microtubules (“spindle”) in one corner of an elongated nucleus. The spindle extends between largely intranuclear “spindle pole bodies”. It rapidly increases in lenght and in maximally elongated nuclei runs in a straight line between the two poles, almost completely embedded in nucleolar material.
It is tentatively proposed that the chromosomes are attached to the poles of the spindle.
The shortness and peripheral, excentric position of developing spindles in already considerably elongated nuclei suggests that the spindlesdoes not actively initiate nuclear elongation but follows it passively. Its function is seen as that of a device for keeping daughter sets of newly formed chromatids rigidly apart during the course of their distribution to daughter nuclei.
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This work has been supported by a grant to C.F. Robinow by the Medical Research Council of Canada.
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McCully, E.K., Robinow, C.F. Mitosis in Mucor hiemalis . Archiv. Mikrobiol. 94, 133–148 (1973). https://doi.org/10.1007/BF00416688
- Electron Microscopy
- Electron Micrographs
- Nuclear Envelope