The sparks are associated with eggs expelling the mineral zinc after they’re injected with spermlike enzymes. (Federal law prohibits researchers from fertilizing eggs with actual sperm.) The flashes, which appear as a halo around each egg, happen a few minutes after fertilization.
Previous mouse experiments demonstrated that these flashes were more intense for higher-quality fertilized eggs. If these “fertility fireworks” are also indicative of egg quality in humans — a question the Northwestern team intends to investigate — doctors could non-invasively select the best eggs for in vitro fertilization.