Oil droplet movement during ooplasmic segregation in the Oryzias latipes (medaka) fish egg

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Abstract

During ooplasmic segregation in the medaka egg, one easily identifiable class of ooplasmic inclusions (oil droplets) segregate to the vegetal pole. In this study, we used time-lapse video microscopy to monitor closely the movements of these droplets in eggs oriented with either their animal pole or their vegetal pole uppermost with respect to gravity. After fertilization, the droplets moved first toward the animal pole and then toward the vegetal pole. In eggs oriented with their vegetal pole uppermost, oil droplets stopped moving toward the animal pole significantly sooner than droplets in eggs oriented with their animal pole uppermost. Moreover, the position of an oil droplet on a meridian connecting the animal pole and vegetal pole of the egg was a factor in determining when that droplet ceased moving toward the animal pole, with droplets nearer the animal pole stopping before droplets farther away from this pole. These results, together with others from our laboratory, suggest that at least three forces act upon oil droplets during ooplasmic segregation in the medaka egg: i) a buoyant force that causes droplets to float toward the top of the egg, ii) a force that restrains oil droplets from floating toward the top of the egg, and iii) a force that actively moves droplets toward the vegetal pole.

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