Figure 10.

(A) Proposed model of spindle morphogenesis. The centrosomes separate to the prophase steady state determined by the balance of a constant outward dynein force and the sum of the inward Ncd force and nuclear elastic force. (B) Snapshot of the computer simulation. The yellow curve above shows the predicted level of dynein activation. The red shading of the cortex indicates the predicted graded actin density (bright, high density; dark, low density). Note that the nucleus is deformed and pressed against the actin cortex. (C) Hypothesis I: Depletion of “inner” astral MTs decreases the inward dynein force on centrosomes but increases the inward Ncd force. However, short ipMT bundles limit the Ncd force when the centrosomes are close, but, as centrosomes separate, the Ncd force grows to match the outward force generated by dynein at the cortex. (D) Hypothesis II: MT tips locally inhibit dynein at the cortex. In the vicinity of the centrosomes and at the spindle equator, MT tips from both poles overlap, so there is a high effective density of MT tips, and dynein is inhibited. In the regions distal to the centrosomes, MT tips from only one pole reach the cortex, so there is a low effective density of MT tips, and dynein is active. Note that dynein is active where it can generate the outward forces.