Abstract
When the anterior-posterior axis of the mouse embryo becomes explicit at gastrulation, it is almost perpendicular to the long uterine axis. This led to the belief that the uterus could play a key role in positioning this future body axis. Here, we demonstrate that when the anterior-posterior axis first emerges it does not respect the axes of the uterus but, rather, the morphology of the embryo. Unexpectedly, the emerging anterior-posterior axis is initially aligned not with the long, but the short axis of the embryo. Then whether the embryo develops in vitro or in utero, the anterior-posterior axis becomes aligned with the long axis of embryo just prior to gastrulation. Of three mechanisms that could account for this apparent shift in anterior-posterior axis orientation-cell migration, spatial change of gene expression, or change in embryo shape-lineage tracing studies favor a shape change accompanied by restriction of the expression domain of anterior markers. This property of the embryo must be modulated by interactions with the uterus as ultimately the anterior-posterior and long axes of the embryo align with the left-right uterine axis. The emerging anterior-posterior axis relates to embryo morphology rather than that of the uterus. The apparent shift in its orientation to align with the long embryonic axis and with the uterus is associated with a change in embryo shape and a refinement of anterior gene expression pattern. This suggests an interdependence between anterior-posterior gene expression, the shape of the embryo, and the uterus.
Highlights
The anterior-posterior axis of the mouse embryo becomes morphologically explicit at embryonic day (E) 6.5
While the mouse embryo appears radially symmetrical at E5.5, embryonic patterning is evident along the proximal-distal axis, with extraembryonic ectoderm located proximally, epiblast distally, and visceral endoderm enveloping both tissues
The Embryo Undergoes Dynamic Changes in Its Shape and Orientation with Respect to the Uterus between Implantation and Gastrulation We first sought to determine the extent to which the morphological axes of the embryo relate to the axes of the uterus shortly after implantation. To address this question we recovered embryos at the earliest possible postimplantation stage (E5.0) up to the time of gastrulation (E6.5) and determined first the extent to which the embryonic region of the developing egg cylinder departed from radial symmetry
Summary
The anterior-posterior axis of the mouse embryo becomes morphologically explicit at embryonic day (E) 6.5. Posterior polarity after implantation appear at E5.5 and are revealed by the asymmetric expression of several genes along the proximal-distal axis of the egg cylinder [1]. While the mouse embryo appears radially symmetrical at E5.5, embryonic patterning is evident along the proximal-distal axis, with extraembryonic ectoderm located proximally, epiblast distally, and visceral endoderm enveloping both tissues. Within these tissues, gene expression patterns further define subdomains of asymmetry along the proximal-distal axis. The emergence of molecular pathways determining axial organization of the postimplantation embryo can be detected along the proximal-distal axis at this stage
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