Embryonic development is a tightly regulated series of events that begins with fertilization of an egg by a single sperm. This remarkable process is surprisingly error prone, with an estimated one out of two conceptions ending in miscarriage. Many miscarriages are caused by early developmental defects arising before pregnancy is perceptible. Carlson and colleagues study the rapid molecular interactions that occur immediately after a sperm fertilizes an egg and ensure successful progression to embryogenesis. By applying biophysical techniques to study fertilization, they capture these ephemeral signaling events.
Polyspermy, fertilization of an egg by more than one sperm, is a significant barrier to successful reproduction. Polyspermy induces embryonic-lethal chromosomal abnormalities in nearly all species. As such, eggs have multiple mechanisms to combat the entry of additional sperm into a fertilized egg. Externally fertilizing animals employ a mechanism called the fast block, which is evoked by fertilization and induces an immediate depolarization of the egg membrane; this change in membrane potential prevents sperm from entering an already-fertilized egg. Using Xenopus laevis as their model organism, Carlson and colleagues have identified the signaling events that control this depolarization. Intriguingly, they have found that the fast block is remarkably similar to pathways that regulate human physiological processes, including vascular tone or excitability of peripheral neurons.