When a sperm penetrates it, an egg releases a wave of stored calcium ions. This surge is the universal signal telling an egg to divide and develop. But what exactly is it about the sperm's arrival that triggers this calcium message?
It's a squirt of nitric oxide from the sperm, report Richard C. Kuo of Stanford University School of Medicine and his colleagues in the Aug. 10 Nature.
Nitric oxide, or NO, has earned high marks in recent years as a versatile signaling molecule used by cells. Kuo and his colleagues recently discovered that sea urchin sperm, especially the heads, contain an enzyme that can synthesize the gas. They further found that this enzyme goes into action just as a sperm infiltrates an egg. The nitric oxide produced triggers the release of calcium, which prompts the egg to make more nitric oxide and free more calcium.