| Abstract: | Morphological changes in the mitotic cytoskeleton (MC) that occurred through the course of the first cleavage of Strongylocentrotus droebachiensis, cultured at 8°C or at 0°C, temperatures within the natural range for this species, have been investigated. Electron microscopy of MCs isolated from zygotes has revealed that they consisted largely of microtubules (mts). Thus, the morphology of these MCs is derived from the arrangement of the mts which form them. During anaphase, astral rays elongated while kinetochore fibers shortened. Asters enlarged during anaphase as a result of two events: astral ray lengthening and centrosphere enlargement. At the end of anaphase, asters of 8°C MCs filled the entire cell volume. The pattern of changes that occurred in 8°C mitotic apparatuses (MAs) also occurred in 0°C MCs. The observation of asters in 0°C MCs is contradictory to that of Stephens ('72b), who reported that 0°C MCs in this species were anastral. However, in 0°C metaphase MCs, the astral rays and spindle fibers were not as long as those in 8°C MCs. Also in 0°C MCs, the centrosphere was largely filled with dense material, whereas the centrosphere in 8°C MCs was larger and contained little dense material. Asters of 0°C MCs did not attain a large enough size to fill the egg volume completely, as did asters of 8°C MCs. |
