Fig. 1.

(A) MT-based dynamics in a live single-celled C. elegans embryo visualized with GFP-tagged β-tubulin (Movie S1). (Left to Right, Top to Bottom) Migration of the male and female pronuclei, pronuclear meeting, NCC centration, and spindle orientation following NCC rotation. Scale bar, 10 μm. (B) Schematic of the biophysical model, with an ellipsoidal cell boundary encapsulating the cytoplasm (yellow), pronucleus (blue), and associated MTs (green). Asymmetric lengths of MTs due to cessation of growth at the cortex creates a force imbalance favoring centration. (Upper Inset) The relationship between load F_m and velocity Δv for a single motor protein. (Lower Inset) Motor proteins embedded in the cytoplasm exert equal and opposite forces ± F_mp on the cytoplasm and MT, resulting in their relative motion.