Asymmetry in the Mitotic Spindle Induced by the Attachment to the Cell Surface during Maturation in the Starfish Oocyte

In order to study the dynamic behavior of the mitotic apparatus leading to unequal cleavage, we investigated the distribution of mitotic microtubules (MTs) during maturation division of starfish oocytes. When the mitotic apparatus attached to the cell surface at metaphase, in both the first and second meiotic division, it is revealed, by immunofluorescence, that the MT distribution in the spindle, as well as in the aster, became asymmetric. MTs in the peripheral half spindle increased in number compared with those in the inner half spindle. Furthermore, these results were confirmed in the living cell by polarization microscopy; shortly after the attachment, the birefringence retardation of the peripheral half spindle became greater than that of the inner one, and the difference increased with time during anaphase. By inhibiting the attachment of the mitotic apparatus by means of centrifugation, the MT distribution maintained a symmetrical pattern through mitosis. These results suggest that the attachment of the mitotic apparatus to the cell surface induces the asymmetrical distribution of MTs not only in the aster but also in the spindle. Such a rich distribution of MTs in the peripheral half spindle appears to ensure chromosome exclusion into the polar body by anchoring them firmly to the cell surface of the animal pole.