Fig. 1. Cortical Ca²⁺ wave pacemakers in the ascidian and mouse egg. (A) Sperm-triggered Ca²⁺ waves in ascidians: the meiotic Ca²⁺ waves, composed of a fertilization wave (F) followed by repetitive Ca²⁺ waves, are initiated by two pacemakers (MI PM_asc and MII PM_asc). (B) An artificial pacemaker (artPM_asc, red arrowhead) can be induced in the animal pole of the egg (a) by global UV photorelease of cgPtdIns(4,5)P₂. The Ca²⁺ waves emitted by this pacemaker are preceded by a pacemaker Ca²⁺ rise (red asterisks). (C) The mouse egg is fertilized at metaphase II and thus possesses only a MII pacemaker (MII PM_mouse). After the fertilization wave (F) starting from the point of sperm entry, repetitive calcium waves emanate from the vegetal cortex of the egg. Each calcium wave is preceded by a pacemaker calcium rise (red asterisks). (D) The drawings represent schematically the organization in the ascidian (MI, MII and artPM_asc) and mouse egg (MII PM_mouse) of the ER network (in red) and mitochondria (in green). Red arrows indicate the direction of the waves, whereas the postulated sites of Ins(1,4,5)P₃ production are symbolized as purple dots. sa: sperm aster; cp: contraction pole. (E) Postulated temporal variations of [Ins(1,4,5)P₃]_c, which may underlie the activity of the meiotic Ca²⁺ wave pacemakers. Two possibilities remain for the ascidian and mouse MII pacemakers: a sustained Ins(1,4,5)P₃ production (pink trace) or oscillatory Ins(1,4,5)P₃ production (purple trace).