SURFACE ULTRASTRUCTURE OF PADDLEFISH EGGS BEFORE AND AFTER FERTILIZATION

The surface ultrastructure of eggs of the paddlefish Polyodon spathula was investigated by scanning electron microscopy. Mature eggs of padd lefish possess four to 12 micropyles in the animal polar region. There are sperm entry sites in the egg surface under the micropyles which c onsist of tufts of microvilli. Five to nine sperm entry sites were obs erved on mature eggs. Probably, the number of sperm entry sites corres ponds to the number of micropyles. In a few eggs, 1 min after fertiliz ation the ball-like enlarged top of a cytoplasmic process (probably a full-grown fertilization cone) had reached the external aperture or th e canal of several micropyles. In other micropyles of the same egg, a few smaller cytoplasmic processes or flocculent material were found in the micropylar canal. With one exception, no sperm tails were found t here. The formation of the full-grown cytoplasmic process is possibly initiated before the cortical reaction has started in an area of the a nimal hemisphere. Three, 10 and 20 min after fertilization, the uneven surface of the cortical cytoplasm in the animal polar region rose gen tly where microvilli were much less than the in other area and togethe r with a secondary polar body at the latter stage. Taken together, pad dlefish eggs may have sperm entry sites corresponding to the number of micropyles and respond to the stimulus of fertilization by forming a few cytoplasmic processes - fertilization cones (larger and smaller). Sperm penetration into the egg may be achieved at an earlier stage of fertilization (sperm-egg contact), as inferred from the fact that a se condary polar body was formed at the 20-min stage irrespective of the exceptional finding of the sperm tail. (C) 1997 The Fisheries Society of the British Isles.