A B S TRACT The early events of fertilization that precede and cause activation of an egg have not been fully elucidated. The earliest electrophysiological change in the sea urchin egg is a sperm-evoked increase of the egg's membrane conductance. The resulting depolarization facilitates entry of the fertilizing sperm and precludes the entry of supernumerary sperm. The sequence of the increase in the egg's membrane conductance, gamete membrane fusion, egg activation, and sperm entry, including causal relationships between these events, are not known. This study reports the use of whole egg voltage clamp and loose patch clamp to monitor simultaneously changes of membrane conductance and capacitance at the site of sperm-egg contact. Measurements were made during sperm-egg interactions where sperm entry readily proceeded or was precluded by maintaining the egg's membrane potential either at large, negative values or at positive values. Whenever the sperm evoked an increase of the egg's membrane conductance, that increase initiated abruptly, was localized to the site of sperm attachment, and was accompanied by a simultaneous abrupt increase of the membrane capacitance. This increase of capacitance indicated the establishment of electrical continuity between gametes (possibly fusion of the gametes' plasma membranes). If sperm entry was blocked by large negative membrane potentials, the capacitance cut off rapidly and simultaneously with a decrease of the membrane conductance, indicating that electrical continuity between gametes was disrupted. When sperm entry was precluded by positive membrane potentials, neither conductance nor capacitance increased, indicating that sperm entry was halted before the fusion of membranes. A second, smooth increase of capacitance was associated with the exocytosis of cortical granules near the sperm in eggs that were activated. Electrical continuity between the gametes always preceded activation of the egg, but transient electrical continuity between the gametes alone was not always sufficient to induce activation. Early during fertilization of sea urchin eggs, the fertilizing sperm attaches to the egg and then, after a brief (10-30 s) latent period (Allen and Griffin, 1958), the egg begins to activate. The early events that lead up to and cause activation have not been fully elucidated.In the sea urchin egg, an increase of membrane conductance that occurs within seconds after sperm attachment is the earliest physiological event indicating intimate interaction between the sperm and the egg. The increase of conductance induced by the sperm depolarizes the egg from a resting value near -70 mV (Chambers and de Armendi, 1979;Lynn and Chambers, 1984;Lynn, McCulloh, and Chambers, 1988). Depolarization initiates an activation potential consisting of three distinct phases (Chambers and de Armendi, 1979). Phases 1 and 2 maintain the membrane potential more positive than + 10 mV as the fertilization envelope elevates over the surface of the egg. Phase 3 returns the membrane potential to a value ...
