Calcium signals and mitochondria at fertilisation

At fertilisation, Ca2+ signals activate embryonic development by stimulating metabolism, exocytosis and endocytosis, cytoskeletal remodelling, meiotic resumption and recruitment of maternal RNAs. Mitochondria present in large number in eggs have long been thought to act as a relay in Ca2+ signalling at fertilisation. However, only recently have studies on ascidians and mouse proven that sperm-triggered Ca2+ waves are transduced into mitochondrial Ca2+ signals that stimulate mitochondrial respiration. Mitochondrial Ca2+ uptake can substantially buffer cytosolic Ca2+ concentration and the concerted action of heterogeneously distributed mitochondria in the mature egg may modulate the spatiotemporal pattern of sperm-triggered Ca2+ waves. Regulation of fertilisation Ca2+ signals could also be achieved through mitochondrial ATP production and mitochondrial oxidant activity but these hypotheses remain to be explored. A critically poised dynamic interplay between Ca2+ signals and mitochondrial metabolism is stimulated at fertilisation and may well determine whether the embryo can proceed further into development. The monitoring of Ca2+ signals and mitochondrial activity during fertilisation in living zygotes of diverse species should confirm the universality of the role for sperm-triggered Ca2+ waves in the activation of mitochondrial activity at fertilisation.