Cortical contraction and ooplasmic movement in centrifuged or artificially constricted eggs ofCiona intestinalis.

To discover the force causing bipolar ooplasmic segregation just after fertilization in ascidian eggs (Ciona intestinalis), cortical contraction and cytoplasmic movement were examined by centrifugation and by artificial constricting techniques. In the centrifuged eggs, the surface constriction appeared independently of cytoplasmic stratification. The yolk layer and the sub-centripetal layer moved toward the vegetal pole in the peripheral region. In the eggs which were artificially constricted by partially broken chorion and then fertilized, the inner cytoplasm always flowed from the vegetal sphere into the animal sphere during bipolar segregation. The direction of this cytoplasmic movement was independent of sphere size. This shows that the force causing cytoplasmic movement is supplied by the contraction of the vegetal-side cortex. It is suggested that the contracting cortex pushes the inner cytoplasm toward the animal pole and drags the peripheral cytoplasm toward the vegetal pole.

The cortical contraction related to the ooplasmic segregation inCiona intestinalis eggs.

The egg cytoplasm of ascidian,Ciona intestinalis, segregates towards both the animal and vegetal poles within a few minutes of fertilization or parthenogetic activation with ionophore A23187. A constriction appears first on the egg surface near the animal pole and then moves to the vegetal pole. Carmine granules and spermatozoa attached to the egg surface move towards the vegetal pole with the movement of the constriction. Microvilli, which are distributed uniformly in unfertilized egg, disappear on the animal side of the constriction and became more dense on the vegetal side of the constriction. Transmission electron microscopy revealed that sub-cortical cytoplasm, containing numerous mitochondria and sub-cortical granules, moves towards the vegetal pole with the movement of the constriction and then concentrates into a cytoplasmic cap at the vegetal pole. An electron-dense layer appears in the cortex of the cap. The ooplasmic segregation and the cortical contraction were inhibited by cytochalasin B and induced by ionophore A23187. These observations suggest that ooplasmic segregation is caused by the cortical contraction which is characterised by a surface constriction and by the formation of an electron-dense layer.