Reproductive functions are changed by microgravity, for example, decreased gravity causes decreases in sperm counts and plasma FSH in males 
and increases in abortion and changes in maternal behavior 
. At the cellular and subcellular levels, microgravity changes cell structure, metabolism, behavior and survival 
. In ground-based studies, the random positioning machine (PRM, a 3D clinostat system), and RCCS are commonly used to simulated microgravity condition. In addition to simulate microgravity, RCCS can be used to generate shear stress to cultured cells 
. Microgravity condition simulated by RCCS promotes spermatocytes entry into the first meiotic division 
. In this study, we used the RCCS to simulated microgravity and examined the effects of simulated microgravity on mouse oocytes maturation. Even though microgravity has no effect on the resumption of meiosis or microfilament organization and functions, microgravity profoundly inhibited the polar body extrusion by disrupting the organization of microtubules. As the γ-tubulin is essential for regulating microtubule dynamics and the abnormal distribution of γ-tubulin in simulated microgravity, we speculated that the disorganization of microtubules in simulated microgravity even not all but partially was the results of the disorder of γ-tubulin.
Microtubules are essential component of meiotic spindle, which manages polar body extrusion and distributes genetic materials equally into the polar body and the remaining oocyte. Microtubule nucleation is initiated from γ-tubulin and disrupted γ-tubulin promotes spindle microtubule disorganization 
. As the oocytes begin to mature, microtubules polymerize around the chromosomes and eventually assemble into the meiotic spindle. Previous studies have showed that microtubule self-organization is gravity-dependent. For human lymphocytes (Jurkat), human epithelial cells (MCF-7), human thyroid carcinoma cells, and swine testicular cells 
, microtubule organizations under simulated microgravity conditions were substantially reduced compared with those under 1 g conditions. In our study, normal meiotic spindles were formed in mouse oocytes under 1 g gravity. But most oocytes cultured in simulated microgravity formed abnormal spindles with few microtubules. Knockdown of γ-tubulin by siRNA in mouse oocytes resulted in smaller spindles with misaligned chromosomes 
. In this study, the organization of γ-tubulin was disordered in simulated microgravity, which induced the failure of aggregation of microtubules and resulted in abnormal spindle. The results indicate that the microtubule polymerization and organization of mouse oocytes was disrupted by simulated microgravity and resulted in the malformation of intact meiotic spindles. Although the chromosomes migration was not inhibited by simulated microgravity most likely through the actions of microfilaments, the homologous chromosomes can not separate without the function of spindle microtubules. So the oocyte maturation was inhibited by the simulated microgravity through disordered γ-tubulin microtubules organization and disrupting meiotic spindle assembly.
During maturation the oocytes develop cell polarity, which is required for asymmetric cell divisions to extrude polar bodies. Microfilaments are required for chromosomes or spindle migration to the cortex of oocytes and are essential for oocyte polarization. A gravity sensitivity of the actin cytoskeleton has been reported on mouse MC 3T3-E1 osteoblasts 
and human breast cancer MCF-7 cells 
. Therefore, we supposed that the oocytes might not polarize themselves under microgravity condition. However, our results showed that the function and organization of microfilaments had not been disrupted by simulated microgravity. Chromosomes could still migrate to oocyte periphery and could induce cortical reorganization of microfilaments to form actin caps as normal gravity. Our results were in accord with the researches on J-111 cells and human SH-SY5Y neuroblastoma, in which cells microfilaments are not affected by microgravity 
. We reasoned that in different cells types there might be different sensitivities of microfilaments in response to microgravity, or that different cells switch on and off different pathways to react microgravity.
One important finding in our study is that simulated microgravity induces cytoplasmic micro-protrusions of maturing oocytes. 12.96%±3.2% (n
1,726) oocytes cultured under simulated microgravity were induced cytoplasmic blebbing. This phenotype was rarely observed in 1 g gravity. Until now, available data concerning structural changes during mouse oocyte maturation in simulated microgravity were relatively scarce. Choroidal epithelial cells appear cytoplasmic extensions on apical surface, partial loss of cell polarization in weightlessness and induced alteration in the fine structure of choroids plexus 
. This was the only one study with cytoplasmic extension in weightlessness we can search. The actin cap was not remarkably in these blebbing oocytes should be because at around anaphase I, oocytes without normal spindle structure can not released from SAC, and microfilaments of actin cap spreaded to the whole oocytes membrane, then induced the cytoplasmic blebbing.
Some animals studies had been conducted on the effect of microgravity on female reproduction, most of these studies focused on the fertilization, embryo development and pregnancy. Fertilization can occur normally under microgravity and preimplantation embryo development might require 1 g environment 
. Rats kept in space during the latter half of their pregnancies are able to support the growth and development of their gestating offspring 
. As female astronauts suppress their menstrual cycles during spaceflight 
, relatively little is known about the effect of spaceflight on female reproduction. Whether the oocyte maturation and ovulation are normal during female astronauts in weightlessness is still unclear. Here, our observations suggest that oocytes maturation were compromised under simulated microgravity environment.