OOPLASMIC SEGREGATION IN THE MEDAKA (ORYZIAS-LATIPES) EGG

Using time-lapse video microscopy, we found that ooplasmic inclusions in the fertilized medaka egg displayed two types of movement during ooplasmic segregation. The first manifested itself as the movement of many inclusions (diameter = 1.5-11 mum) toward the animal pole at about 2.2 mum min-1; this type of movement appeared to be streaming. The second type of movement was faster (about 44 mum min-1) and saltatory; inclusions displaying this type of movement were smaller (diameter less-than-or-equal-to 1.0 mum) and moved toward the vegetal pole. The movement of oil droplets toward the vegetal pole of the egg may represent a third type of motion. All these movements began only after a strong contraction of the ooplasm toward the animal pole, which at 25-degrees-C began 10-12 min after fertilization and <3 min after formation of the second polar body. In eggs treated with microtubule poisons-colchicine, colcemid, or nocodazole-oil droplets did not move toward the vegetal pole, saltatory motion toward the vegetal pole was absent, and the growth of the blastodisc was slowed. Eggs treated with beta-lumicolchicine, an inactive derivative of colchicine, showed normal movements. Colchicine, while not inhibiting formation of the second polar body, did inhibit pronuclear migration. These results suggest that microtubules are involved in the movement of some ooplasmic inclusions, including oil droplets, toward the vegetal pole; the movement of ooplasmic inclusions toward the animal pole; and pronuclear migration.