Signs of the principle body axes prior to primitive streak formation in the rabbit embryo.

An early common element during anterior-posterior axis formation amongst amniotes is the primitive streak, running longitudinally in the two-layered embryonic disc. In mammals the primordium of this transient structure is the first definite morphological sign of the anterior-posterior axis, while in avian embryos the axis is visible and apparently defined earlier. Here we scrutinize suggestions that in mammals also there are earlier signs of axis formation by using correlative low- and high-resolution light microscopy on tissues from rabbit embryos at 6.3 and 6.5 days post-conception, i.e. immediately before and after primitive streak formation. A series of semithin sections were cut from resin-embedded embryonic discs that had been photographed previously at low power. In embryos at 6.5-days post-conception the primitive streak is as long as up to half the diameter of the embryonic disc, extending anteriorly from a thickening, here called the posterior node, at the posterior margin, which contains the first mesoderm cells ingressing from the epiblast. On both sides of the primitive streak there is a triangular area that appears light in surface views of fixed embryos and correlates with stretches of low-columnar simple epithelium in an otherwise high-columnar pseudostratified epiblast. Within the anterior margin, which has a sharper contour than the rest of the circumference of the embryonic disc, there is a narrow, crescent-shaped dark zone caused by increased cellular height and number in both epiblast and hypoblast. These characteristics of the anterior margin are also found at 6.3 days post-conception, at which stage there is no sign of a primitive streak or a posterior node. The posterior margin, in contrast, is ill-defined in these earlier embryos, or there is a light crescent within the posterior margin, which has the same histological characteristics as the bilateral posterior triangular areas of primitive streak stages. Because the anterior differentiation occurs prior to primitive streak formation and is a sign of both the anterior-posterior and the transverse axes of the embryonic disc, and because some of its histological characteristics are found in primate and human embryos, we propose to name this structure the 'anterior marginal crescent' and to add it to the list of transient structures that gradually establish the principal body axes in mammals. The anterior manifestation of body axes in mammals is thus essentially different from axis development in the avian embryo, where differentiation of these axes is first manifest at the posterior margin.