Competitive inhibition by Rauber's sickle of the primitive streak and/or (pre)neural plate inducing effects of sickle endoblast in avian blastoderms.

When in unincubated chicken blastoderms the Rauber's sickle is (sub)totally mechanically removed by selective scraping, the further evolution of the blastoderm in culture is often profoundly disturbed, going from only expansion of the upper layer and preneural plate formation to the development of a slowly growing miniature embryo. Our results suggest that the developmental potencies of the embryo are related to the presence or absence of Rauber's sickle material left after its removal. This can be checked after culture by the presence or nonpresence of junctional endoblast (derived from Rauber's sickle) and the concomitant induction of blood islands in the immediate neighborhood. Our study thus indicates that without Rauber's sickle (in the cases of successful total selective removal), an avian blastoderm cannot develop normally, even in the presence of an intact caudal marginal zone. After placing a fragment of quail sickle endoblast on the anti-sickle region of unincubated chicken blastoderms from which the Rauber's sickle was (sub)totally removed, different developmental scenarios were seen, according to the degree of removal, both in the anti-sickle as in the sickle regions. 1) If Rauber's sickle activity is strongly reduced, then besides a centripetally directed miniature embryo, induced by the remnants of the autochthonous Rauber's sickle, an additional centripetally directed embryo or preneural plate (without accompanying blood islands) develops in the anti-sickle region under inductory influence of the apposed quail sickle endoblast. We make a distinction between a neural plate and a preneural plate. The latter consists of a thickening of the upper layer (with the same initial aspect as a neural plate) adjacent to endophyll or sickle endoblast in the absence of chordomesoblast and gastrulation phenomena. 2) If Rauber's sickle activity is totally absent, then the inducing power of the sickle endoblast fragment becomes maximal and, starting from the anti-sickle region, one single embryo (without blood islands) extending over the whole area centralis appears. 3) If much of the Rauber's sickle material has been left in the blastoderm, then the inducing activity of the sickle endoblast, placed on the anti-sickle region, will be totally suppressed (although the sickle endoblast remains intact) and neither a preneural plate nor a primitive streak was induced. After placing a fragment of quail sickle endoblast on the anti-sickle region of an unincubated chicken blastoderm from which the Rauber's sickle and surrounding tissues were completely excised, an embryo was always induced by the sickle endoblast in the adjacent upper layer of this anti-sickle region. In the absence of sickle endoblast, this never occurred. Thus, our experiments demonstrate that in the absence of the Rauber's sickle, a parent tissue (sickle endoblast) induces both gastrulation and neurulation phenomena, while in the full presence of Rauber's sickle these functions are totally suppressed. Moreover, Rauber's sickle not only organizes gastrulation and blood island formation by itself but also influences neurulation at a distance (in space and time) by part of its cell lineage (i.e., sickle endoblast). Our study suggests that the inhibitory effect of Rauber's sickle on its parent tissue (sickle endoblast) represents an early mechanism impairing polyembryony, so that only a single primary major organizer (Rauber's sickle) remains active in the young avian germinal disc.