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.[Keywords: Mouse preimplantation embryo; egg; embryonic axis; polarity; blastocoel; blastocyst] Supplemental material is available at http://www.genesdev.org. Various studies on experimentally manipulated embryos, including blastomere isolation (Tarkowski 1959;Tarkowski and Wroblewska 1967;Rossant 1976) and chimera formation (Tarkowski 1961), have demonstrated the highly regulative capacity of mammalian preimplantation embryos. This led to the long-held assumption that the polarity of most mammalian embryos remains undetermined until the blastocyst stage. However, recent studies (Gardner 1997(Gardner , 2001Fujimori et al. 2003) have proposed that the embryonic-abembryonic (Em-Ab) axis of the mouse blastocyst arises perpendicular to the first cleavage plane. The second polar body (2pb) was used as a stationary marker for the "animal pole (A-pole)" during preimplantation development, and the first cleavage plane was considered to be meridional with only "occasional" exceptions (Gardner 1997(Gardner , 2001, which led to the conclusion that polarity of the mouse embryo is predetermined in the egg (Gardner 1997(Gardner , 2001, as is the case for most nonmammalian species. Nonmeridional first cleavage (not passing through the 2pb) (Gardner and Davies 2003a) and movement of the 2pb toward the first cleavage furrow (Gray et al. 2004) were also observed, although the significance of these observations was not fully recognized. It has also been reported that the sperm entry position (SEP) determines the embryonic axis (Piotrowska and ZernickaGoetz 2001); in this case, the first cleavage plane is defined as passing through both the 2pb and SEP. Moreover, it has been suggested that the blastomere in the two-cell embryo that inherits the SEP will divide earlier than the other and contribute preferentially to the inner cell mass (ICM) lineage (Piotrowska and Zernicka-Goetz 2001), although other authors ...
The mechanism of embryonic polarity establishment in mammals has long been controversial. Whereas some claim prepatterning in the egg, we recently presented evidence that mouse embryonic polarity is not established until blastocyst and proposed the mechanical constraint model. Here we apply computer simulation to clarify the minimal cellular properties required for this morphology. The simulation is based on three assumptions: (1) behavior of cell aggregates is simulated by a 3D vertex dynamics model; (2) all cells have equivalent mechanical properties; (3) an inner cavity with equivalent surface properties is gradually enlarged. However, an initial attempt reveals a requirement for an additional assumption: (4) the surface of the cavity is firmer than intercellular surfaces, suggesting the presence of a basement membrane lining the blastocyst cavity, which is indeed confirmed by published data. The simulation thus successfully produces a structure recapitulating the mouse blastocyst. The axis of the blastocyst, however, remains variable, leading us to an additional assumption: (5) the aggregate is enclosed by a capsule, equivalent to the zona pellucida in vivo. Whereas a spherical capsule does not stabilize the blastocyst axis, an ellipsoidal capsule eventually orients the axis in accordance with its longest diameter. These predictions are experimentally verified by time-lapse recordings of mouse embryos. During simulation, equivalent cells form two distinct populations composed of smaller inner cells and larger outer cells. These results reveal a unique feature of early mammalian development: an asymmetry may emerge autonomously in an equivalent population with no need for a priori intrinsic differences.
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.
Summary Human pluripotent stem cells (hPSCs) have proven to be valuable tools for both drug discovery and the development of cell-based therapies. However, the long non-coding RNA XIST , which is essential for the establishment and maintenance of X chromosome inactivation, is repressed during culture, thereby causing erosion of dosage compensation in female hPSCs. Here, we report that the de novo DNA methyltransferases DNMT3A/3B are necessary for XIST repression in female hPSCs. We found that the deletion of both genes, but not the individual genes, inhibited XIST silencing, maintained the heterochromatin mark of H3K27me3, and did not cause global overdosage in X-linked genes. Meanwhile, DNMT3A/3B deletion after XIST repression failed to restore X chromosome inactivation. Our findings revealed that de novo DNA methyltransferases are primary factors responsible for initiating erosion of dosage compensation in female hPSCs, and XIST silencing is stably maintained in a de novo DNA-methylation-independent manner.
Global demethylation of DNA which marks the onset of development occurs asynchronously in the mouse; paternal DNA is demethylated at the the zygote stage, whereas maternal DNA is demethylated later in development. The biological function of such asymmetry and its underlying mechanisms are currently unknown. To test the hypothesis that the early demethylation of male DNA may be associated with protamine-histone exchange, we ,used round spermatids, whose DNA is still associated with histones, for artificial fertilization (round spermatid injection or ROSI), and compared the level of methylation of metaphase chromosomes in the resulting zygotes with the level of methylation in zygotes obtained after fertilization using mature sperm heads (intracytoplasmic sperm injection or ICSI). In contrast to ICSI-derived zygotes, ROSI-derived zygotes possessed only slightly demethylated paternal DNA. Both types of zygotes developed to term with similar rates which shows that hypomethylation of paternal DNA at the zygotic metaphase is not essential for full development in mice. Incorporation of exogenously expressed histone H2BYFP into paternal pronuclei was significantly higher in ICSI-derived zygotes than in ROSI-derived zygotes. Surprisingly, in the latter the incorporation of histone H2BYFP into the paternal pronucleus was still significantly higher than into the maternal pronucleus, suggesting that some exchange of chromatin-associated proteins occurs not only after ICSI but also after ROSI. This may explain why after ROSI, some transient demethylation of paternal DNA occurs early after fertilization, thus providing support for the hypothesis regarding the link between paternal DNA demethylation and protamine/histone exchange.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.