The Site of Spermatozoon Entrance in the Unfertilized Egg of Discoglossus pictus (Anura): An Electron Microscope Study

Abstract

The egg of Discoglossus pictus has an animal dimple, a pit located at the animal hemisphere pole, which is the only site successfully penetrated by the sperm. Ultrastructural analysis of the unfertilized egg surface reveals unique features characterizing the animal dimple. They are: 1) regularly spaced, finger-like microvilli supported by bundles of fibers which appear to be microtubules and surrounded by antennular glycocalyx, present at the surface of the animal dimple. The orientation and distribution of the fibers suggest that they may play an essential role in maintenance of the shape of the animal dimple: 2) granules present in the peripheral cytoplasm of the animal dimple whose characteristics are somewhat different from those of the typical Anuran cortical granules and which are absent from the rest of the peripheral cytoplasm of the egg. The unique presence of these granules at the animal dimple and their apparent loss at the time of fertilization suggest that they might actively participate in the fertilization process.