Fig. 9. Model depicting the nuclear localization and release of sperm-derived Ca²⁺-releasing activity. At fertilization, the sperm introduces a Ca²⁺-releasing activity. This activity, which may be a PLC or an activator or substrate of PLC (see text), is depicted by black dots or black shading. After fertilization, the Ca²⁺-releasing activity is proposed to localize to the pronuclei (dark stippling). The nuclear localization inhibits the ability to generate Ins(1,4,5)P₃ and so the Ca²⁺ oscillations stop. Other factors also appear to be at play to desensitize Ins(1,4,5)P₃-induced Ca²⁺ release in pronucleate embryos, as depicted by the grey shading of the cytoplasm (see text for more details). The pronuclei migrate to the centre of the embryo and NEBD takes place, marking the start of the first mitotic division. NEBD leads to the factor responsible for Ca²⁺-releasing activity to disperse in the cytoplasm, where it has the capacity to generate Ca²⁺ transients. The oscillations stop again at the two-cell stage when the nuclei form. This model of nuclear compartmentalization of Ca²⁺-releasing activity, including PLCs, may be important for regulating mitotic Ca²⁺ transients in a variety of cells (see text).