Interactions of the meiotic spindle with mitotic chromosomes in GV mouse oocytes

JAK2 inhibitor CEP-33779 prevents mouse oocyte maturation in vitro

The inhibitor CEP-33779 is a specific selective inhibitor of Janus kinase 2 (JAK2). In most somatic cells, JAK2 plays essential roles in cellular signal transduction and in the regulation of cell cycle. Little is known regarding the effects of JAK2 on mammalian oocyte maturation. In the present study, we investigated the effects of CEP-33779 on mouse oocytes’ meiosis and the possible mechanisms of JAK2 during mouse oocyte maturation. We detected the distribution of JAK2 during the mouse oocyte maturation. The results showed that JAK2 was mainly distributed in the cytoplasm during maturation. We cultured mouse oocytes with CEP-33779, examined the maturation rate, spindle morphology, and organization of microfilaments during the mouse oocyte maturation. While the rate of germinal vesicle breakdown (GVBD) did not differ between the treated and control groups, the rate of oocyte maturation decreased significantly when treated with CEP-33779. The rate of maturation was 21.14% in treated group and was 81.44% in control group. The results show that CEP-33779 inhibits the maturation of mouse oocytes. There was no obvious difference in the meiotic spindle morphology between the treated and control groups. The results show that CEP-33779 treatment did not disrupt the reorganization of microtubules. The microfilament observation shows that the microfilament did not form actin cap and the spindle stayed at the center of the oocyte in the treated group. CEP-33779 treatment inhibited the maturation of mouse oocytes which might be because of the disruption of formation of the actin cap. These results suggest that JAK2 regulated the microfilaments aggregation during the mouse oocyte maturation.

Meiosis I in Xenopus oocytes is not error-prone despite lacking spindle assembly checkpoint

The spindle assembly checkpoint, SAC, is a surveillance mechanism to control the onset of anaphase during cell division. SAC prevents anaphase initiation until all chromosome pairs have achieved bipolar attachment and aligned at the metaphase plate of the spindle. In doing so, SAC is thought to be the key mechanism to prevent chromosome nondisjunction in mitosis and meiosis. We have recently demonstrated that Xenopus oocyte meiosis lacks SAC control. This prompted the question of whether Xenopus oocyte meiosis is particularly error-prone. In this study, we have karyotyped a total of 313 Xenopus eggs following in vitro oocyte maturation. We found no hyperploid egg, out of 204 metaphase II eggs with countable chromosome spreads. Therefore, chromosome nondisjunction is very rare during Xenopus oocyte meiosis I, despite the lack of SAC.

Polar body emission

Generation of a haploid female germ cell, the egg, consists of two rounds of asymmetric cell division (meiosis I and meiosis II), yielding two diminutive and nonviable polar bodies and a large haploid egg. Animal eggs are also unique in the lack of centrioles and therefore form meiotic spindles without the pre-existence of the two dominant microtubule organizing centers (centrosomes) found in mitosis. Meiotic spindle assembly is further complicated by the unique requirement of sister chromatid mono-oriented in meiosis I. Nonetheless, the eggs appear to adopt many of the same proteins and mechanisms described in mitosis, with necessary modifications to accommodate their special needs. Unraveling these special modifications will not only help understanding animal reproduction, but should also enhance our understanding of cell division in general.

Simulated microgravity compromises mouse oocyte maturation by disrupting meiotic spindle organization and inducing cytoplasmic blebbing

In the present study, we discovered that mouse oocyte maturation was inhibited by simulated microgravity via disturbing spindle organization. We cultured mouse oocytes under microgravity condition simulated by NASA's rotary cell culture system, examined the maturation rate and observed the spindle morphology (organization of cytoskeleton) during the mouse oocytes meiotic maturation. While the rate of germinal vesicle breakdown did not differ between 1 g gravity and simulated microgravity, rate of oocyte maturation decreased significantly in simulated microgravity. The rate of maturation was 8.94% in simulated microgravity and was 73.0% in 1 g gravity. The results show that the maturation of mouse oocytes in vitro was inhibited by the simulated microgravity. The spindle morphology observation shows that the microtubules and chromosomes can not form a complete spindle during oocyte meiotic maturation under simulated microgravity. And the disorder of γ-tubulin may partially result in disorganization of microtubules under simulated microgravity. These observations suggest that the meiotic spindle organization is gravity dependent. Although the spindle organization was disrupted by simulated microgravity, the function and organization of microfilaments were not pronouncedly affected by simulated microgravity. And we found that simulated microgravity induced oocytes cytoplasmic blebbing via an unknown mechanism. Transmission electron microscope detection showed that the components of the blebs were identified with the cytoplasm. Collectively, these results indicated that the simulated microgravity inhibits mouse oocyte maturation via disturbing spindle organization and inducing cytoplasmic blebbing.

Enucleated GV oocytes as recipients of embryonic nuclei in the G1, S, or G2 stages of the cell cycle

Universal recipients in the G2 phase of mitotic cell cycle (preactivated oocytes, zygotes, blastomeres) accept embryonic nuclei in all the stages of their cell cycle. To test if recipients in the G2 of meiotic cycle (immature oocytes) are universal recipients, mouse germinal vesicle (GV) oocytes were enucleated and reconstructed with blastomere nuclei in the G1, S, or G2 stages. Analysis of their maturation has shown that about 30% of the G1 nuclei and 60% of G2 nuclei allow for normal metaphase II (MII), both in the oocytes with and without the first polar body (1st PB). Among oocytes reconstructed with the S phase nuclei, only 8% or less have normal MII, although 75% of them extrude 1st PB. No phase of donor cell cycle prevented the abnormal acceleration of 1st PB extrusion, found in reconstructed GV oocytes. In conclusion, enucleated GV oocytes are not universal recipients of embryonic nuclei, because they do not accept the S donors. However, both the G1 and G2 donor nuclei can be reprogrammed in the GV oocyte cytoplasm.

Association of spindle midzone particles with polo-like kinase 1 during meiosis in mouse and human oocytes

Polo-like kinase 1 (Plk1) has been reported to localize to the spindle midzone during meiosis in mouse oocytes. However, it has not been reported in human oocytes. In this study, the interaction of the meiotic structures and chromosome segregation in mouse and human oocytes were studied by time-lapse differential interference contrast microscopy. Using immunocytochemical studies, the localization of polo-like kinase 1 and its association with microtubules were examined during the extrusion of first and second polar bodies. It was found that Plk1 was localized in the spindle midzone in human oocytes at anaphase I and telophase I. Also, three-dimensional confocal laser microscopy showed that the meiotic spindle midzone contained numerous dot-like particles that were stained by anti-Plk1 antibody. These particles were aligned in the plane of the meiotic midzone in mouse and human oocytes.

Artificial gametes

In vitro fertilization (IVF) has been an efficient medical treatment for infertility in the past decades. However, conventional IVF approaches may be insufficient when gametes are lacking or non-viable thus precluding a significant number of patients from treatment. Ultimately, creation of artificial gametes may provide an universal solution for all indications. Somatic cell nuclear transfer (SCNT) has provided successful cloning in different animal species indicating that a derived technology may be applicable in infertility treatment procedures. Attempts to produce functional male or female gamete through nuclear transfer have been described through the process called haploidization. Initial successes have been observed, however, significant alterations at spindle construction and chromosomal segregation were also described. Stem cell technology may provide an alternative route to obtain fully functional gametes. Both sperm cells and oocytes were obtained using specific culture conditions for embryo originated stem cell. These two mainstream approaches are presented in the current review. Both of these techniques are involving sophisticated methods and consequently both of them demonstrate technical and ethical challenges. Related questions on (mitotic/meiotic) cell division, genetic/epigenetic alterations and cell renewal are needed to be addressed before clinical application.