Mitotic Spindles and Cleavage Planes Are Oriented Randomly in the Two-Cell Mouse Embryo

Abstract

Most experimental embryological studies performed on the early mouse embryo have led to the conclusion that there are no mosaically distributed developmental determinants in the zygote and early embryo (for example see [1-6]). It has been suggested recently that "the cleavage pattern of the early mouse embryo is not random and that the three-dimensional body plan is pre-patterned in the egg" (in [7] for review see [8-10]). Two major spatial cues influencing the pattern of cleavage divisions have been proposed: the site of the second meiotic division [11, 12] and the sperm entry point [13-14], although the latter is controversial [15-17]. An implication of this hypothesis is that the orientations of the first few cleavage divisions are stereotyped. Such a define cleavage pattern, leading to the segregation of developmental determinants, is observed in many species [18]. Recently, it was shown that the first cleavage plane is not predetermined but defined by the topology of the two apposing pronuclei [19]. Because the position of the female pronucleus is dependent upon the site of polar body extrusion and the position of the male pronuclei is dependent upon the sperm entry point [19-20], this observation leaves open the possibility that the sperm may provide some kind of directionality [7]. But, even if asymmetries were set up only after fertilization, a stereotyped cleavage pattern should take place during the following cleavage divisions. Thus, we studied the cleavage pattern of two-cell embryos by videomicroscopy to distinguish between the two hypotheses. After the mitotic spindle formed, its orientation did not change until cleavage. During late metaphase and anaphase, the spindle poles appear to be anchored to the cortex through astral microtubules and PARD6a. Only at the time of cleavage, during late anaphase, do the forming daughter cells change their relative positions. These studies show that cleavage planes are oriented randomly in two-cell embryos. This argues against a prepatterning of the mouse embryo before compaction.

Computer simulation of emerging asymmetry in the mouse blastocyst

“…Central questions include how this apparently initial asymmetry is established in the early mouse embryo, and whether the underlying mechanism in this process is comparable to that operating in non-mammalian 'model' organisms, in which localized ('prepatterned') determinants play an essential role. Whereas recent reports (Gardner, 1997;Gardner, 2001;PiotrowskaNitsche et al, 2005;Torres-Padilla et al, 2007) claim the presence of 'prepatterning' in the mouse egg and preimplantation embryos reminiscent of non-mammals, our studies (Hiiragi and Solter, 2004;Motosugi et al, 2005) and those by others (Alarcon and Marikawa, 2003;Alarcon and Marikawa, 2005;Chroscicka et al, 2004;Kurotaki et al, 2007;Louvet-Vallee et al, 2005) provide evidence supporting a 'regulative' and 'mechanical constraint' (Alarcon and Marikawa, 2003;Kurotaki et al, 2007;Motosugi et al, 2005) model of blastocyst morphogenesis. In this model, the first embryonic polarity, the Em-Ab axis, is not established until the blastocyst cavity is localized at one end (the abembryonic pole), and its eventual position is determined by the spatial constraints imposed by the zona pellucida (ZP) in conjunction with the epithelial seal in the outer cell layer.…”

Developmental capacity and pregnancy rate of tetrahedral- versus non-tetrahedral-shaped 4-cell stage human embryos

“…Other studies in mice suggested no prelocalization of developmental determinants in the early blastomeres and showed that the second polar body is not a reliable morphogenetic determinant for the first cleavage [47,48]. Some claim that there is no rotation of the spindle itself but a movement of the blastomeres [49]. From our study, it can be concluded that a non-tetrahedral arrangement of the blastomeres within the 4-cell stage human embryo may affect its subsequent development.…”

Long-Term, Six-Dimensional Live-Cell Imaging for the Mouse Preimplantation Embryo That Does Not Affect Full-Term Development

“…In our previous paper [14], even though mRNA encoding solely EGFP was injected at a concentration of 1 mg/ml, 36% of embryos could reach to morula stage. Moreover, although the design of the mRNA injected (coding sequences, UTRs and poly(A) tail) was different, other groups used 0.5 mg/ml of mRNA for injection, and it seems that morula/ blastocyst development was not affected [5,26]. Thus, the toxicity to embryonic development appears to depend on the types and amounts of exogenous proteins translated from the mRNA injected rather than the mRNA itself.…”