The Blood Island is a Site of Formation of the Primary Embryo Thrombocyte in the Chick Blastoderm

Using the immunohistological technique we inquired at what developmental stage and in which site of chick blastoderm does the embryo thrombocyte (ET) begin to differentiate. An anti-ET antibody was raised against rabbits by injecting ETs isolated from blood of 10 day chick embryos. By applying the indirect staining method to smear preparations of blood collected from developing embryos it was confirmed that cytoplasm of the ET showed more intense staining than that of the erythroid cell and that the ET population could be distinguished from the erythrocyte population by this antibody. Cells showing the intense staining could be detected first in blood islands of the area opaca vasculosa of stage 9+ blastoderms. These embryo thromboblasts were found singly or in groups of a small number at dorsal periphery of cell clusters in the blood island. The electron microscopy revealed that embryo thromboblasts appeared in the same position in the stage 9+ blastoderm. At stage 10+ or later embryo thromboblasts were also present adhering to the vascular endothelium or free in the vessel lumen. We conclude that ETs start differentiating from primitive mesenchymal cells localized in the blood island of the area opaca vasculosa at stage 9 or earlier, migrate thereafter to vessel lumen, and enter the blood stream.