Abstract
Meiotic maturation is essential for the reproduction procedure of many animals. During this process an oocyte produces a large egg cell and tiny polar bodies by highly asymmetric division. In this study, to fully understand the sophisticated spatiotemporal regulation of accurate oocyte meiotic division, we focused on the global and local changes in the tension at the surface of the starfish (Asterina pectinifera) oocyte in relation to the surface actin remodeling. Before the onset of the bulge formation, the tension at the animal pole globally decreased, and started to increase after the onset of the bulge formation. Locally, at the onset of the bulge formation, tension at the top of the animal pole began to decrease, whereas that at the base of the bulge remarkably increased. As the bulge grew, the tension at the base of the bulge additionally increased. Such a change in the tension at the surface was similar to the changing pattern of actin distribution. Therefore, meiotic cell division was initiated by the bulging of the cortex, which had been weakened by actin reduction, and was followed by contraction at the base of the bulge, which had been reinforced by actin accumulation. The force generation system is assumed to allow the meiotic apparatus to move just under the membrane in the small polar body. Furthermore, a detailed comparison of the tension at the surface and the cortical actin distribution indicated another sophisticated feature, namely that the contraction at the base of the bulge was more vigorous than was presumed based on the actin distribution. These features of the force generation system will ensure the precise chromosome segregation necessary to produce a normal ovum with high accuracy in the meiotic maturation.
Highlights
The oocyte reduces the number of chromosomes to half through two successive meiotic cell divisions, in which the oocyte produces a normal ovum with high accuracy
Oocyte meiotic divisions are characterized by highly asymmetric division that occurs through dynamic surface actin remodeling [1,2,3,4,5,6]
To fully understand the sophisticated spatiotemporal regulation of accurate oocyte meiotic divisions, it is important to focus on the spatiotemporal changes in the tension at the surface in relation to the surface actin remodeling
Summary
The oocyte reduces the number of chromosomes to half through two successive meiotic cell divisions, in which the oocyte produces a normal ovum with high accuracy. It has been shown that mechanical factors, including tensions at the surface, affect surface actin remodeling to dictate dramatic shape changes [9]. To fully understand the sophisticated spatiotemporal regulation of accurate oocyte meiotic divisions, it is important to focus on the spatiotemporal changes in the tension at the surface in relation to the surface actin remodeling. In the starfish oocyte meiotic divisions, global and local actin remodeling has been observed [10]. In spatiotemporal regulation of oocyte meiotic divisions, actin remodeling on various spatial scales from a tiny polar body scale to a larger whole oocyte scale is important. Further studies on the tensions at the surface on various spatial scales in relation to the actin remodeling are expected to unveil the mechanism of accurate oocyte meiotic divisions
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