MICROTUBULE-BASED MOVEMENTS DURING OOPLASMIC SEGREGATION IN THE MEDAKA FISH EGG (ORYZIAS-LATIPES)

We used time-lapse video microscopy to monitor the effects of cytochal asin D (CCD) and demecolcine on cytoplasmic streaming toward the anima l pole of the medaka egg, the formation of the blastodisc at the anima l pole, the movement of oil droplets in the cytoplasm toward the veget al pole, and the saltatory movement of small cytoplasmic parcels towar d the animal pole and vegetal pole. Cytochalasin D inhibited both cyto plasmic streaming toward the animal pole and the formation of the blas todisc, suggesting a role for microfilaments in these processes. Howev er, CCD had no apparent effect on saltatory movement or on the movemen t of oil droplets toward the vegetal pole. Thus, the segregation of oi l droplets toward the vegetal pole is not the result of the bulk movem ent of ooplasm toward the animal pole. In eggs treated with demecolcin e, oil droplets did not move toward the vegetal pole but instead float ed to the uppermost portion of the egg, and saltatory movement was abs ent, suggesting that microtubules are required for these movements. Th e effects of demecolcine on oil droplet movement and saltatory movemen t could be reversed by irradiating the eggs with UV light (360 nm). Us ing indirect immunofluorescence, we showed that irradiation of demecol cine-treated eggs with UV light regenerated microtubules within the ir radiated region. The specificity of the mechanism responsible for the vegetal poleward movement of oil droplets was assessed by microinjecti ng droplets of five other fluids-mineral oil, silicone oil, vegetable oil, and two fluorinated aliphatic compounds-into the ooplasm. None of these fluids segregated with the endogenous oil droplets. These resul ts suggest that a specific mechanism, probably involving microtubules, is responsible for the segregation of oil droplets to the vegetal pol e.