Changes in the chorion and sperm entry into the micropyle during fertilization in the teleostean fish, Oryzias latipes.

Specific antibodies against the major chorionic glycoproteins (ZI1-2 and ZI3) of unfertilized eggs were used to analyze the differences in the chorion and its surrounding constituents before and after fertilization. The glycoproteins in the inner layers of the chorion and its surrounding material were specifically stained by both of the antibodies. Thirty and 60 min after activation, the thickness of the chorion's inner layers was already reduced and the micropylar canal was closed. At the same time, the broadly diluted mucous area (DMA) of glycoproteins on the outermost layer of the chorion in unfertilized eggs was modified to a thin, compact layer. When unfertilized eggs were treated with trypsin, the inner third portion of the micropylar canal closed and the glycoproteins in the DMA were digested. The incidence of sperm entry into the micropyle of these eggs was extremely reduced. These results suggest that in medaka eggs, the chorionic glycoproteins in the DMA on the chorion surface, which have an affinity for spermatozoa, play an important role in sperm guidance into the micropyle.