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

Abstract

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 ...

Cdx2 Expression Regulates Trophectoderm Lineage Specification in Mouse Embryos

“…The results published by Roberts group are in direct conflict with the data recently published by Motosugi and colleagues [6] who used timelapse video microscopy and fluorescent cell labeling to demonstrate a lack of patterning in preblastocyst mouse embryos. Multiple conflicting reports have been published on this subject [2][3][4][5][6].…”

“…Multiple conflicting reports have been published on this subject [2][3][4][5][6]. While it is not clear at this stage what accounts for the differences in the results obtained by various groups, one possible explanation is variation in the mouse strains used to obtain embryos.…”

“…While in a large number of organisms it is clear that the destiny of the blastomeres is predetermined in the egg, there is still debate over when blastomere commitment occurs during mammalian development. Multiple studies have addressed this issue using a variety of approaches, including labeling individual blastomeres with membrane soluble dye [2] or β-galactosidase reporter under CAG-control in a Cre-LoxP system [3], labeling zona pellucida of the embryo with oil drops [4], time-lapse and confocal microscopy [5,6]. However, such efforts have produced conflicting results.…”

Cell and molecular regulation of the mouse blastocyst

“…The earlier-dividing two-cell blastomere did not give rise to any trophectoderm. However, this proposal contradicts all previously published observations regarding developmental potential and fate of cells from the two-cell embryo, because all other groups have reported contribution of both two-cell blastomeres to both ICM and TE (Gardner, 2001;Piotrowska et al, 2001;Marikawa, 2003, 2005;Chroscicka et al, 2004;Hiiragi and Solter, 2004;Plusa et al, 2005a;Motosugi et al, 2005;PiotrowskaNitsche and Zernicka-Goetz, 2005). Before these observations can be considered as changing the paradigms of early mouse development, it is essential to explore whether this lineage relationship holds true in other mouse strains and if it can be reproduced by other groups using alternate lineage tracing methods.…”

The Leading Blastomere of the 2-Cell Stage Parthenogenetic Porcine Embryo Contributes to the Abembryonic Part First

“…Unlike fertilized oocytes, no tendency for the first two parthenogenetic blastomeres to follow different fates was found, supporting the conclusion that the fertilization process contributes to setup of embryonic patterning. In contrast with the above claims, recent time-lapse recordings have shown that the first cleavage plane is not predetermined but is defined only by the topology of the two apposing pronuclei in the mouse embryo [9] and that the polarity of the mouse embryo is established at the blastocyst stage and is not prepatterned [18,19]. Controversy still exists as to whether the individual blastomeres from twocell-stage embryos have identical developmental fates.…”