SIGNS OF THE PRINCIPLE BODY AXES PRIOR TO PRIMITIVE STREAK FORMATION IN THE RABBIT EMBRYO

An early common element during anterior-posterior axis formation among st amniotes is the primitive streak, running longitudinally in the two -layered embryonic disc. In mammals the primordium of this transient s tructure is the first definite morphological sign of the anterior-post erior axis, while in avian embryos the axis is visible and apparently defined earlier. Here we scrutinize suggestions that in mammals also t here 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 prim itive streak formation. A series of semithin sections were cut from re sin-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, extendin g 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 tr iangular area that appears light in surface views of fixed embryos and correlates with stretches of low-columnar simple epithelium in an oth erwise high-columnar pseudostratified epiblast. Within the anterior ma rgin, which has a sharper contour than the rest of the circumference o f the embryonic disc, there is a narrow, crescent-shaped dark zone cau sed by increased cellular height and number in both epiblast and hypob last. These characteristics of the anterior margin are also found at 6 .3 days post-conception, at which stage there is no sign of a primitiv e streak or a posterior node. The posterior margin, in contrast, is il l-defined in these earlier embryos, or there is a light crescent withi n the posterior margin, which has the same histological characteristic s as the bilateral posterior triangular areas of primitive streak stag es. Because the anterior differentiation occurs prior to primitive str eak formation and is a sign of both the anterior-posterior and the tra nsverse axes of the embryonic disc, and because some of its histologic al 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 princ ipal body axes in mammals. The anterior manifestation of body axes in mammals is thus essentially different from axis development in the avi an embryo, where differentiation of these axes is first manifest at th e posterior margin.