The Egg-to-Embryo Transition

Egg activation is the transformation of a quiescent egg or egg cell into a new individual. Elevated intracellular cortical calcium is the earliest event of egg activation and is responsible for triggering a series of milestones that initiate early development. Although mitochondrial respiration and protein synthesis are reactivated almost immediately following a temporary rise in intracellular calcium levels, development of the newly formed individual cannot proceed until the paternal and maternal haploid genomes merge and replicate. Degradation of unneeded parental components and initiation of the transcription of zygotic genes, known as zygotic genome activation, are also orchestrated during the earliest stages of development that transition a fertilized egg or egg cell to an early embryo.