Biomedical subjects

Zbigniew Polanski

Publications and source records attributed to Zbigniew Polanski.

6 recordsLinked to original sources

PubMed2006

Space asymmetry directs preferential sperm entry in the absence of polarity in the mouse oocyte.

Knowledge about the mechanism that establishes embryonic polarity is fundamental in understanding mammalian development. In re-addressing several controversial claims, we recently proposed a model in which mouse embryonic polarity is not specified until the blastocyst stage. Before fertilization, the fully differentiated oocyte has been characterized as "polarized," and we indeed observed that the sperm preferentially enters the polar body half. Here we show that preferential sperm entry is not due to an intrinsic polarity of the oocyte, since fertilization takes place uniformly when the zona pellucida is removed. We suggest that the term "asymmetry" denotes morphological differences, whereas "polarity" in addition implies developmental consequences. Thus, the mouse oocyte can be considered "asymmetric" but "non-polarized." The penetration through the zona pellucida is also random, and a significant proportion of sperm binds to the oocyte membrane at a point distant from the zona penetration site. Time-lapse recordings confirmed that sperm swim around the perivitelline space before fertilization. Experimental enlargement of the perivitelline space in the non-polar body half increased the regional probability of fertilization. Based on these experiments, we propose a model in which the space asymmetry exerted by the first polar body and the zona pellucida directs sperm entry preferentially to the polar body half, with no need for oocyte polarity.

Animals↗

PubMed2006

Germinal vesicle material drives meiotic cell cycle of mouse oocyte through the 3'UTR-dependent control of cyclin B1 synthesis.

We compared the profile of histone H1 kinase activity, reflecting Maturation Promoting Factor (MPF) activity in oocytes bisected at the germinal vesicle (GV) stage and allowed to mature as separate oocyte halves in vitro. Whereas the oocyte halves containing the nucleus exhibited the same profile of increased kinase activity as that typical for intact oocytes, the anuclear halves revealed strong inhibition of the increase in this activity soon after germinal vesicle breakdown (GVBD). In contrast, the profile of MAP kinase activity did not differ significantly between anuclear and nucleus-containing oocyte halves throughout maturation. Of the two MPF components, CDK1 and cyclin B1, the amount of the latter was significantly reduced in anuclear halves, a reduction due to low-level synthesis and not to enhanced degradation. Expression of three reporter luciferase RNAs constructed, respectively, to contain cyclin B1-specific 3'UTR, the globin-specific 3'UTR, or no 3'UTR sequence was enhanced in nuclear halves, with significantly greater enhancement for the construct containing cyclin B1-specific 3'UTR as compared to the two other RNAs. We conclude that the profile of activity of MPF during mouse oocyte maturation is controlled by an unknown GV-associated factor(s) acting via 3'UTR-dependent control of cyclin B1 synthesis. These results require the revision of the hitherto prevailing view that the control of MPF activity during mouse oocyte maturation is independent of GV-derived material.

3' Untranslated Regions↗

PubMed2005

The first mitosis of the mouse embryo is prolonged by transitional metaphase arrest.

The first mitosis of the mouse embryo is almost twice as long as the second. The mechanism of the prolongation of the first mitosis remains unknown, and it is not clear whether prometaphase or metaphase or both are prolonged. Prometaphase is characterized by dynamic chromosome movements and spindle assembly checkpoint activity, which prevents anaphase until establishment of stable kinetochore-microtubule connections. The end of prometaphase is correlated with checkpoint inactivation and disappearance of MAD2L1 (MAD2) and RSN (CLIP-170) proteins from kinetochores. Spindle assembly checkpoint operates during the early mouse mitoses, but it is not clear whether it influences their duration. Here, we determine the length of prometaphases and metaphases during the first two embryonic mitoses by time-lapse video recording of chromosomes and by immunolocalization of MAD2L1 and RSN proteins. We show that the duration of the two prometaphases does not differ and that MAD2L1 and RSN disappear from kinetochores very early during each mitosis. The first metaphase is significantly longer than the second one. Therefore, the prolongation of the first embryonic mitosis is due to a prolonged metaphase, and the spindle assembly checkpoint cannot be involved in this process. We show also that MAD2L1 staining disappears gradually from kinetochores of oocytes arrested at metaphase of the second meiotic division. This shows a striking similarity between the first embryonic mitosis and metaphase arrest in oocytes. We postulate that the first embryonic mitosis is prolonged by a transient metaphase arrest that is independent of the spindle assembly checkpoint and is similar to metaphase II arrest. The molecular mechanism of this transient arrest remains to be elucidated.

Animals↗

PubMed2005

Oocyte nucleus controls progression through meiotic maturation.

We analyzed progression through the meiotic maturation in oocytes manipulated to replace the prophase oocyte nucleus with the nucleus from a cumulus cell, a pachytene spermatocyte or the pronucleus from a fertilized egg. Removal of the oocyte nucleus led to a significant reduction in histone H1 kinase activity. Replacement of the oocyte nucleus by a pronucleus followed by culture resulted in premature pseudomeiotic division and occasional abnormal cytokinesis; however, histone H1 kinase activity was rescued, microtubules formed a bipolar spindle, and chromosomes were condensed. In addition to the anomalies observed after pronuclear transfer, those after transfer of the nucleus from a cumulus cell or spermatocyte included a dramatically impaired ability to form the bipolar spindle or to condense chromosomes, and histone H1 kinase activity was not rescued. Expression of a cyclin B-YFP in enucleated oocytes receiving the cumulus cell nucleus rescued histone H1 kinase activity, but spindle formation and chromosome condensation remained impaired, indicating a pleiotropic effect of oocyte nucleus removal. However, when the cumulus cell nucleus was first transformed into pronuclei (transfer into a metaphase II oocyte followed by activation), such pronuclei supported maturation after transfer into the oocyte in a manner similar to that of normal pronuclei. These results show that the oocyte nucleus contains specific components required for the control of progression through the meiotic maturation and that some of these components are also present in pronuclei.

Animals↗

PubMed2005

Polarity of the mouse embryo is established at blastocyst and is not prepatterned.

Polarity formation in mammalian preimplantation embryos has long been a subject of controversy. Mammalian embryos are highly regulative, which has led to the conclusion that polarity specification does not exist until the blastocyst stage; however, some recent reports have now suggested polarity predetermination in the egg. Our recent time-lapse recordings have demonstrated that the first cleavage plane is not predetermined in the mouse egg. Here we show that, in contrast to previous claims, two-cell blastomeres do not differ and their precise future contribution to the inner cell mass and/or the trophectoderm cannot be anticipated. Thus, all evidence so far strongly suggests the absence of predetermined axes in the mouse egg. We observe that the ellipsoidal zona pellucida exerts mechanical pressure and space constraints as the coalescing multiple cavities are restricted to one end of the long axis of the blastocyst. We propose that these mechanical cues, in conjunction with the epithelial seal in the outer cell layer, lead to specification of the embryonic-abembryonic axis, thus establishing first polarity in the mouse embryo.

Animals↗

PubMed2004

The spindle assembly checkpoint is not essential for CSF arrest of mouse oocytes.

In Xenopus oocytes, the spindle assembly checkpoint (SAC) kinase Bub1 is required for cytostatic factor (CSF)-induced metaphase arrest in meiosis II. To investigate whether matured mouse oocytes are kept in metaphase by a SAC-mediated inhibition of the anaphase-promoting complex/cyclosome (APC/C) complex, we injected a dominant-negative Bub1 mutant (Bub1dn) into mouse oocytes undergoing meiosis in vitro. Passage through meiosis I was accelerated, but even though the SAC was disrupted, injected oocytes still arrested at metaphase II. Bub1dn-injected oocytes released from CSF and treated with nocodazole to disrupt the second meiotic spindle proceeded into interphase, whereas noninjected control oocytes remained arrested at metaphase. Similar results were obtained using dominant-negative forms of Mad2 and BubR1, as well as checkpoint resistant dominant APC/C activating forms of Cdc20. Thus, SAC proteins are required for checkpoint functions in meiosis I and II, but, in contrast to frog eggs, the SAC is not required for establishing or maintaining the CSF arrest in mouse oocytes.

Anaphase-Promoting Complex-Cyclosome↗