Rotation of Meiotic Spindle Is Controlled by Microfilaments in Mouse Oocytes1

Abstract

The completion of meiosis requires the spatial and temporal coordination of cytokinesis and karyokinesis. During meiotic maturation, many events, such as formation, location, and rotation of the meiotic spindle as well as chromosomal movement, polar body extrusion, and pronuclear migration, are dependent on regulation of the cytoskeleton system. To study functions of microfilaments in meiosis, we induced metaphase II (MII) mouse oocytes to resume meiosis by in vitro fertilization or parthenogenetic activation, and we treated such oocytes with cytochalasin B (CB). The changes of the meiotic spindle, as visualized in preparations stained for beta-tubulin and chromatin, were observed by fluorescent confocal microscopy. The meiotic spindle of MII oocytes was observed to be parallel to the plasmalemma. After meiosis had resumed, the spindle rotated to the vertical position so that the second polar body could be extruded into the perivitelline space. When meiosis resumed and oocytes were treated with 10 micro g/ml of CB, the spindle rotation was inhibited. Consequently, the oocyte formed an extra pronucleus instead of extruding a second polar body. These results indicate that spindle rotation is essential for polar body extrusion; it is the microfilaments that play a crucial role in regulating rotation of the meiotic spindle.

Actin cytoskeleton in cell polarity and asymmetric division during mouse oocyte maturation

“…In MII, the spindle assembles with its long axis lying parallel to the overlying cortex and therefore must rotate about 90 in order to orient properly for polar body extrusion. Spindle rotation relies on actin cytoskeleton as it is disrupted by cytochalasin B treatment [Zhu et al, 2003]. Further, the fact that blebbistatin and ML-7 block spindle rotation indicates that myosin II contractility may be required for the process [Matson et al, 2006;Wang et al, 2011].…”

GM130, a cis-Golgi protein, regulates meiotic spindle assembly and asymmetric division in mouse oocyte

“…35,36 Similarly, disrupting the actin cytoskeleton also affects MII spindle rotation upon fertilization or parthenogenetic activation. 37 However, in oocytes treated with GM130 MO-injection, the chromosome component of the spindle, induced an "F-actin cap" in the adjacent cortical region. Clearly, GM130 depletion did not grossly distort the actin.…”

Regulation of dynamic events by microfilaments during oocyte maturation and fertilization

“…Ca 2þ is released from intracellular stores and activates PKC, leading to CG exocytosis (Sun 2003). New studies suggest that CG release in mature eggs is dependent on calciumdependent proteins like those in somatic cells required to undergo calcium-regulated exocytosis (Abbott & Ducibella 2001), and calmodulin-dependent kinase II (CaMKII) may act as a switch in the transduction of the calcium signal (Sun 2003).…”

“…When oocytes are treated with cytochalasin B or D, spindle rotation is inhibited. Consequently, the oocyte forms an extra female pronucleus in the ooplasm instead of extruding a second polar body (Zhu et al 2003). By using Pol-Scope (Cambridge Research & Instrumentation, Boston, MA, USA), Navarro et al (2005) also observed the inhibition of spindle rotation by cytochalasin D in mouse oocytes.…”