New findings on the orientation of the mouse anterior-posterior (A-P) axis before and during the initiation of gastrulation using a more refined embryo staging.

A clinically significant event of early mammalian embryogenesis is the generation and early development of the anterior-posterior (A-P) axis, the imaginary line along which the structures from head to tail will form. This axis not only appears before gastrulation but is also oriented in a specific way in relation to the long and short diameters of the bilaterally symmetric epiblast. In mice, the most widely used mammalian model of early embryogenesis, the A-P axis is normally aligned with the long epiblast diameter by the early streak (ES) stage, a time during early gastrulation around embryonic day 6.