Practice question

Fertilization calcium transient releases calcium principally from intracellular stores rather than influx from seawater, despite high external concentration. Experiments in calcium-free seawater confirm wave persists. Storage organelle is endoplasmic reticulum, particularly cortical ER clustered beneath plasma membrane studded with IP3 receptors and ryanodine receptors. IP3 generated at sperm entry diffuses to ER opening channels via calcium-induced calcium release amplifying wave globally. Mitochondria and nucleus contain calcium but not primary sources. ER sequestration via SERCA pumps reloads stores after activation, preparing embryo for subsequent mitotic calcium transients regulating cell cycle resumption and activation.