Effects of pH on the Fertilization Response of the Medaka Egg

The effects of changing the intra‐ and extracellular pH on the cortical reaction and sperm penetration into eggs were examined in Oryzias latipes. When eggs were inseminated in a saline adjusted to various pH values with different buffers, fertilization took place normally over a wide range of external pH (pH0) from about 6 to 10 with a peak at 7 to 8.5. The primary causes of the reduced fertilization were impaired motility or immobility of the spermatozoa in the acidic saline and the swelling of the chorion in the alkaline saline. A rise in the intracellular pH (pHi) shortly after commencement of the cortical reaction was found by monitoring the color of a pH indicator micro‐injected into the cytoplasm. The experimental results obtained by microinjection of various pH buffers (pH 4–12) suggest that the intracellular membrane fusion of the plasma membrane with the cortical alveolar membrane (CABD) is affected by the acidic pHi while the propagation of the CABD and the intercellular membrane fusion between the plasma membranes of the egg and the spermatozoon are affected by the acidic pH0.