The Determination of Spindle Polarity in Early Mitotic Stages of the Dividing Grasshopper Neuroblasts

Although the spindle body of the grasshopper neuroblasts at the early mitotic stages does not have any mechanical linkage to the surrounding cell cortex, a spindle axis is inevitably oriented in parallel with the original division axis. The present study analyzes how the definite orientation of spindle axis along the cap cell (CC)-ganglion cell (GC) axis of the neuroblast is maintained during these stages by use of the microdissection technique and electron microscopy. After removing a microneedle from the cell, metaphase spindles approximately 90°. rotated were able to return autonomously to the original axis. After the middle anaphase, however, the rotated spindle could not return at all. The electron microscopic observations revealed a characteristic behavior of an electron dense layer (EDL) in the CC-side cortex during neuroblast mitosis. The EDL first appeared at very late prophase and became most conspicuous at metaphase. It became discontinuous by the beginning of middle anaphase and then completely disappeared at middle anaphase. So long as an EDL existed in the CC-side polar cortex, 90° rotated spindle bodies were able to return autonomously to the original axis. After the disappearance of the EDL, the autonomous return of the rotated spindle no longer occurred. From these circumstantial evidence, it is conceivable that the orientation of the spindle body along the CC-GC axis is maintained by the interaction between the EDL and the spindle pole.