Embryonic axis orientation in the mouse and its correlation with blastocyst relationships to the uterus. Part 1. Relationships between 82 hours and 4 1/4 days.

Up until at least 7 1/4 days p.c., the orientation of all axes in the mouse conceptus, embryonic axis included, is directly correlated with two orientations of the 82 h blastocyst within the lumen. These two are: the almost horizontal position of the blastocyst's inner cell mass-abembryonic pole axis and the fixation of its abembryonic pole to either the right wall of the uterine horn (type-R orientation) or to the left (type-L orientation). At 88 h and at 4 1/4 and 6 3/4 - 7 1/4 days equal numbers of conceptuses are found in type R and L orientations. Axes of symmetry also are recognizable in some developmentally advanced 82 h blastocysts and in all older conceptuses. For example, when the inner cell mass-abembryonic axis has become vertically oriented within the lumen by 4 1/4 days, the inner cell mass is oblique to this axis. It is concluded that the 82 h blastocyst's orientation to the uterine walls and floor provides it with positional information used in the location of its axes.