Probing the Functional Role of Spindle Midzone in Cytokinesis by Microperforation

Cao and Wang, Mole. Biol. Cell 7:225-232 (1996)

While some experiments suggest that astral microtubules are primarily responsible for signaling cytokinesis, other observations suggest that spindle midzone may also play a role. The functional role of midzone may be determined by asking if a perforation between the midzone and the cortex, generated by pushing down a blunted microneedle, can block crucial cytokinesis signals from reaching the cortex.

Inhibition of Cortical Ingression by Membrane Perforation Created during Metaphase and Early Anaphase

When the perforation is created during metaphase and the first 1-2 min of anaphase, cleavage occurs only on the side facing the mitotic spindle. The perforation extends into a slit which moves rapidly across the equatorial zone. The cell fails to divide due to the inhibition of ingression on the other side of the perforation. These observations suggest that signals emanating from the chromosomal region are required for the initiation of cytokinesis. Recording time, 12 min.

Lack of Effect of Membrane Perforation Created during Mid-Anaphase

When the perforation is created after the first 1-2 min of anaphase, cleavage occurs on both sides of the perforation. Thus the signal from the chromosomal region must be transient in nature and not continuously required for sustaining cortical ingression. Recording time, 8 min.

Behavior of Membrane Perforation Created outside the Equatorial Zone

Perforation created outside the equatorial plane has no effect on cytokinesis. The perforation migrates toward the center of the spindle and disappears. These observations suggest that cytokinesis signal is highly localized along the equatorial plane and that there is a general cortical/membrane movement toward the central equatorial region. Recording time, 13 min.