Fig. 8. A model for pronuclear assembly, motility and union. Sperm entry activates the mature oocyte, leading to second polar body extrusion (A; first polar body not shown). Formation of pronuclei is accompanied by sperm-aster-independent microtubules (B, left), which bring dynactin (black rectangle with side arm) and vimentin filaments (green lines) to the cytoplasmic face of the nuclear pore complex (basket structure) (B, right). Disassembly of microtubules and inhibition of nucleoporins and dynactin block proper complex formation at the outer surfaces of both pronuclei. Growth of sperm-aster microtubules, nucleated by the centrosome attached to the male pronucleus, extends microtubule plus ends away from the male pronucleus, some of which then reach the female pronuclear surface (C, left). These microtubule plus ends could deliver dynein (red wishbone) preferentially to the surface of the female pronucleus, allowing dynein to bind to dynactin and vimentin at nuclear pores, and enabling the dynein-dynactin complex to transport the female pronucleus to the minus ends along the sperm aster (C, right). The inhibition of either dynactin or dynein blocks migration and prevents apposition (D).