An ultrastructural study of early cleavage in Mytilus Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/05741v46k

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  • Ultrastructural analysis and chemical inhibition studies were carried out on first cleavage of Mytilus zygotes to determine possible relationships between the mitotic apparatus and the cortex during polar lobe formation. The stages of mitosis correlated with the appearance of the polar lobe were first determined by light microscopy. The polar lobe does not appear until anaphase of first cleavage. The early polar lobe is symmetrical in position at the vegetal pole, but at late anaphase it shifts position towards the side of the future CD blastomere. During cytokinesis the furrow at the vegetal pole is displaced by the polar lobe resulting in an unequal division. Ultrastructural study at anaphase shows astral microtubules penetrating the polar lobe. At the animal pole microtubules reach almost to the cortex. When furrowing begins cortical granules move into the furrow region at both the animal and vegetal poles. An amorphous electron dense area is present just below the plasma membrane in all cell furrows. This region may correspond to a contractile band of microfilaments. Cell division inhibitors were used to analyze the role of microtubules and microfilaments in Mytilus cleavage. Prior to anaphase, mercaptoethanol, an inhibitor of microtubule function, halts cell division in Mytilus. Before the mitotic apparatus is formed, mercaptoethanol also prevents polar lobe formation. Polar lobe formation can occur in the inhibitor once the mitotic apparatus has begun to form. Cytochalasin B which destroys microfilaments inhibits both polar lobe formation and cytokinesis in Mytilus. Once the polar lobe has appeared, it is resorbed when the cells are placed in cytochalasin. It is suggested that both microtubules and a contractile band of microfilaments play a role in first cleavage. Initial polar lobe formation could be triggered by microtubules of the mitotic apparatus penetrating the vegetal pole whereas final constriction of the definitive polar lobe and the cleavage furrow may be caused by microfilaments.
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