Abstract
In Discoglossus pictus eggs, the animal dimple is the only place where fertilization can occur. The carbohydrate composition of the dimple membrane was investigated using fluorescent-labeled lectins. Receptors for soybean agglutinin (SBA) and wheat germ agglutinin (WGA) are specifically localized on the dimple walls, whereas fucose-like residues are only present at the bottom of the dimple. This lectin binding pattern is transient since it only appears when the dimple is completely formed and disappears immediately after fertilization. The formation of the dimple requires an increase in surface. Our ultrastructural studies suggest that membrane growth occurs through the insertion of vesicles into the plasma membrane. We hypothesize that the lectin-binding glycocomponents that we observed are inserted along with the vesicles which fuse into the membrane. These vesicles most probably originate from the annulate lamellae and cisternal stacks present in full-grown oocytes. The possibility that the surface features of the dimple, such as the microvilli, the complex glycocalyx, and particulary the neutral carbohydrate residues (fucose-like), might favor sperm-egg fusion is discussed.
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