Sperm-triggered Calcium Oscillations at Fertilization
Résumé
Reinitiation of embryonic development at fertilization is brought about by a sperm-triggered rise in egg cytosolic calcium (Ca 2 +) levels. Three models of how sperm trigger this Ca 2 + increase have been proposed: the conduit, contact and content models of fertilization. Overwhelming evidence indicates that the Ca 2 +-releasing second messenger inositol 1,4,5-trisphosphate (InsP 3) is produced at fertilization. The task ahead will be to determine how egg phospholipase Cr is activated at fertilization to generate InsP 3 • Although sperm trigger the initial Ca 2 + rise in the egg, maternal factors determine whether the Ca 2 + rise will be a solitary transient or else aseries of oscillations. This is important since a solitary Ca 2 + spike within physiological limits is not a sufficient stimulus for meiotic exit in ascidians. It is likely that the egg cyclin B-dependent kinase (Cdkl) activity determines whether the egg displays one Ca 2 + transient or aseries of Ca 2 + oscillations; persistently elevated Cdkl activity prolongs the duration of sperm-triggered Ca 2 + oscillations indefinitely. Partly because of this finding, but also since all the sperm-triggered Ca 2 + oscillations are Ca 2 + waves that display a predictable temporal pattern and spatial organisation, the ascidian has become an excellent testing ground for the various models of fertilization. Cytosolic extracts prepared from A. aspersa sperm injected into homologous eggs mimic fertilization using these three criteria: temporal pattern, spatial organisation, and regulation by egg Cdkl activity. We therefore suggest that the content (sperm factor) model best explains fertilization in ascidians.
Domaines
Biologie cellulaire
Origine : Accord explicite pour ce dépôt