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Formation of the Contractile Ring in Dividing Neuroblasts of Chortophaga

Grasshopper neuroblasts always divide unequally in a definite direction to form small ganglion cells (GC). During mid-anaphase, the mitotic apparatus shifts toward the GC side, and a pole of the spindle anchors to the cell cortex. The eccentric location of the mitotic apparatus induces the cleavage furrow in an off-centered position. This position can be altered artificially by rotating the mitotic apparatus with a microneedle.'.' We succeeded in demonstrating the presence of the contractile ring in the cleavage furrow region in intact neuroblasts, as well as in operated cells in which the mitotic apparatus was rotated 90" at mid-anaphase. To observe intact neuroblasts, the thorax segments of embryos were used. Neuroblasts manipulated in hanging-drop cultures were fixed several minutes after the operation, followed by routine specimen preparation for electron microscopy. In the cell at mid-anaphase, which is 0.5-1 minute before the furrow initiation (26" C), a thin, dense layer appeared in the flattened equatorial cortex. Higher magnification revealed that the microfilaments were aligned perpendicular to the spindle axis in the cortex. The cleavage furrow first appeared in the equatorial cortex at the beginning of late anaphase (FIG. la). The fully formed contractile ring was an electrondense layer, about 100 nm in thickness. In grazing sections of the cleavage furrow, microfilaments 6-7 nm in diameter were parallel to the cleavage plane (FIG. lb). As cleavage proceeded, the contractile ring's diameter and width became reduced, whereas the thickness and density remained unchanged. When the mitotic apparatus was rotated 90" at mid-anaphase, the cell began to elongate along the rotated spindle axis, leaving the cytoplasm on the GC side as a protrusion. The cytoplasmic protrusion was absorbed into the cell body in a short time, and furrowing was initiated around the middle part of the rotated mitotic apparatus (FIG. lc). In a cell fixed 1 minute after the operation (FIG. Id), the cell surface of the protrusion was undulated. An array of microfilaments aligned parallel to the original cleavage plane existed near the base of the protrusion (FIG. le). These microfilaments were judged to be the original contractile ring filaments by their orientation and position. In a cell fixed 4 minutes after the operation (FIG. 2a), part of the cell membrane on the GC side folded, these folds were probably remnants of the protrusion. A small number of microfilaments aligned parallel to the cell surface