Figure 1. Development of the mouse embryo from fertilization to gastrulation. At 3.5 days the blastocyst forms and a clear proximodistal polarity is evident in the conceptus. The ovoid shape of the blastocyst means that there is also an unpolarized axis of bilateral asymmetry. By 4.5 days this axis has acquired polarity due to a distinct tilt in the profile of the inner cell mass (ICM). Two days after implantation and 6.5 days after fertilization, gastrulation commences and the primitive streak forms at the embryonic–extraembryonic junction. Mesoderm (crosshatching) is produced in the streak. The origin of the streak marks the posterior aspect of the future organism. By 7.5 days the streak has elongated to the distal tip of the embryo and its anterior end has formed a specialized structure, the node. Axial mesendoderm emanates from the node and moves anteriorly to underly prospective neurectoderm. The axes of the future organism are explicit at 6.5 days and are shown superimposed on an adult mouse.

Figure 2. Manipulations of preimplantation embryos which suggest that the mammalian egg does not contain essential patterning cues for axis determination. (a) Development of a mouse from a 1/2 blastomere. (b) Isolated 1/4 blastomeres form trophectoderm vesicles devoid of inner cell mass (ICM) but if aggregated with other blastomeres they have the potential to form all the tissues of the blastocyst and of a liveborn mouse. (c) Juxtaposing two eight-cell embryos results in their aggregation and formation of a single giant blastocyst which can give rise to a normal liveborn chimaeric mouse. (d) Normally inside cells of a 16-cell morula tend to form ICM whereas outside cells form trophectoderm. Normal blastocysts can be made from exclusively inside or exclusively outside cells.

Figure 3. Diagram illustrating gene expression and cell lineage studies which indicate that proximal–distal polarity of the implanting mouse embryo is transformed into anterior–posterior polarity by directional cell movement in the visceral endoderm and in the epiblast.