Ionic changes in the mitotic apparatus at the metaphase/anaphase transition

The ultrastructure of an intraspindle membrane system in meiosis of spider spermatocytes

An extensive system of membranes was found in the spindles of spermatocytes of two wolf spider species, Lycosa georgicola and L. rabida. Serial section reconstructions of this membrane system revealed that each meiotic bivalent is encased in a tube of membrane, which encloses both kinetochore microtubule bundles and approaches to within a few microns of the centriolar complex. The membrane tube is open at the polar ends. The membrane composing the tube is doubled and resembles smooth endoplasmic reticulum (ER). No evidence of nuclear pore complexes has been found in the intraspindle membrane system, but typical pores are present on the nuclear envelope of prophase cells. The membrane tubes are fenestrated and microtubules sometimes penetrate these fenestrae. Besides its possible function in the regulation of chromosome movement, the intraspindle membrane system may participate in the nonrandom segregation of the sex chromosomes at meiosis in these spiders.

Control mechanisms of the cell cycle: role of the spatial arrangement of spindle components in the timing of mitotic events

To characterize the control mechanisms for mitosis, we studied the relationship between the spatial organization of microtubules in the mitotic spindle and the timing of mitotic events. Spindles of altered geometry were produced in sea urchin eggs by two methods: (a) early prometaphase spindles were cut into half spindles by micromanipulation or (b) mercaptoethanol was used to indirectly induce the formation of spindles with only one pole. Cells with monopolar spindles produced by either method required an average of 3 X longer than control cells to traverse mitosis. By the time the control cells started their next mitosis, the experimental cells were usually just finishing the original mitosis. In all cases, only the time from nuclear envelope breakdown to the start of telophase was prolonged. Once the cells entered telophase, events leading to the next mitosis proceeded with normal timing. Once prolonged, the cell cycle never resynchronized with the controls. Several types of control experiments showed that were not an artifact of the experimental techniques. These results show that the spatial arrangement of spindle components plays an important role in the mechanisms that control the timing of mitotic events and the timing of the cell cycle as a whole.