The development of heterologous grafts, labeled with thymidine- 3H in the cardiac area of the chick blastoderm

Abstract

Under certain experimental conditions, E + M of the noncardiac area (squares 15–18 of Fig. 1) in stages 5− to 5+, transplanted in the cardiac area of another embryo in the same stage, was incorporated within the host and had the capacity of differentiating into heart tissue (endocardium, epimyocardium) and coelomic mesothelium of the host (Figs. 2–4). The endoderm of the same squares gave rise to the endoderm of the foregut and intestinal canal. During the development there was a displacement between the mesoderm and endoderm, the first displacing in a cranial direction with relation to the second. This demonstrates that in these experimental circumstances grafts of these squares to the heart area can differentiate into structures different from those of their prospective fate and that therefore they are not determined or only partially determined at the time of grafting. In these cases, the heterologous graft continued the morphogenetic movements of the host region and became incorporated within it, resulting in a typical, or almost typical, embryo with a single heart, the same as occurs with a homologous graft, e.g., of a heart area. In the cases in which the graft did not become incorporated within the embryo, the endoderm of the graft formed extraembryonic endoderm of the host but could be recognized, even “ in toto,” because of its different structure (Figs. 6 and 8). Even in these cases, isolated cells could pass on to the host and formed parts of some of its structures, but never as a cohesive sheet of cells as in the cases in which incorporation was complete. Even when the graft was divided into three or four portions, as in the case of embryo 2–68, these fused with themselves and with the host and developed as if they were only one piece, going on to form part of the coelomic mesothelium and of the endocardium and myoepicardium of the heart of the host.