Tension,microtubule rearrangements,and the proper distribution of chromosomes in mitosis

The basis for stable versus unstable kinetochore orientation was investigated by a correlated living-cell/ultrastructural study of grasshopper spermatocytes. Mal-oriented bivalents having both kinetochores oriented to one spindle pole were induced by micromanipulation. Such malorientations are stable while the bivalent is subject to tension applied by micromanipulation but unstable after tension is released. Unstable bivalents always reorient with movement of one kinetochore toward the opposite pole. Microtubules associated with stably oriented bivalents, whether they are mal-oriented or in normal bipolar orientation, are arranged in orderly parallel bundles running from each kinetochore toward the pole. Similar orderly kinetochore microtubule arrangements characterize mal-oriented bivalents fixed just after release of tension. A significantly different microtubule arrangement is found only some time after tension release, when kinetochore movement is evident. The microtubules of a reorienting kinetochore always include a small number of microtubules running toward the pole toward which the kinetochore was moving at the time of fixation. All other microtubules associated with such a moving kinetochore appear to have lost their anchorage to the original pole and to be dragged passively as the kinetochore proceeds to the other pole. Thus, the stable anchorage of kinetochore microtubules to the spindle is associated with tension force and unstable anchorage with the absence of tension. The effect of tension is readily explained if force production and anchorage are both produced by mitotic motors, which link microtubules to the spindle as they generate tension forces.     

The stellate layer and rhythmic contractions of the Oryzias latipes embryo

The objective of this study was to determine which cells in the medaka (Oryzias latipes) embryo participate in the rhythmic contraction waves that propagate slowly across the yolk sac throughout most of embryonic development. To facilitate observation of the cells, we inhibited the contractions temporarily by incubating the embryos with o-nitrobenzylacetate, n-heptanol, or n-octanol. After we washed out the inhibitor, isolated cells in a subepithelial layer (similar to the stellate layer in Fundulus heteroclitus) began to pulse. Stellate cells are much smaller than cells in the surface epithelium (enveloping layer) and are present throughout the developmental period during which the contractions occur, stage 14 to stage 26. We conclude that the active force for the rhythmic contraction waves is provided by cells in the stellate layer and that cells in the enveloping layer are passively deformed by the contraction of cells in the closely apposed stellate layer.     

Physical characterisation of plasmids in a morpholine-degrading mycobacterium

The plasmid content of an environmental mycobacterium (MorG) which degrades morpholine (Mor+ phenotype) was investigated. The combination, in what appears to be a novel way, of restriction endonuclease digestion, gel electrophoresis and scanning densitometry permitted the resolution of mixed plasmid preparations into four distinct plasmids with sizes of 54 (approximately), 27.7, 22.8 and 22.6 kb. These plasmids were named pMOR1, 2, 3 and 4, respectively. The Mor+ phenotype was found to be unstable during acriflavin treatment. In four independently isolated Mor- mutants, plasmid pMOR2 was found to have acquired an insert of approximately 1.8 kb within a specific 5.9 kb BamHI fragment. It is concluded that pMOR2 is involved in the coding of the Mor+ phenotype.     

The Circadian Activity and Body Temperature Rhythms of Mice During Their Last Days of Life

Motor activity and body temperature rhythms have been investigated telemetrically in old mice until their death. The present paper is based mainly on the data of two animals which could be monitored over a sufficient length of time (more than three weeks), though the other three animals showed similar results. The daily body temperature rhythm of old mice was more stable as compared to the activity rhythm and was detectable until the last day of life. Its magnitude, defined as the difference between maximum and minimum, was similar to that obtained in middle-aged mice. By contrast, the activity rhythm disappeared or started to become erratic earlier. Unlike the position in younger animals, the body temperature rhythm was phase delayed with respect to the activity rhythm. In one mouse, the increased instability of the phase position two weeks before its death led to a free run with a period length of about 23.7 h. Both activity and temperature rhythms were fragmented in old mice. In the case of the body temperature this was obviously caused by masking. After purifying the raw temperatures, the fragmentation disappeared. On the other hand, the free-run condition was not caused by masking. The sensitivity of body temperature to motor activity was different from younger mice, and this probably reflects changes/deteriorations in the physiology of thermoregulation during the last days of life. In one mouse, during the last 4 days of life, a sharp, torpor-like decrease of body temperature corresponding with the time of the daily minimum was observed. This phenomenon was not found in other mice, though all of them died during the falling period of the temperature rhythm. The results confirm our hypothesis that the endogenous clock may work even during the very last days of life. The ability to synchronize with the periodic environment deteriorates earlier. Also they suggest that these phenomena are not only typical for the activity rhythm but apply also to the body temperature rhythm.