Origin of aortic cell clusters in the chicken embryo

In 3-day-old embryos the aortic cell clusters formed two parallel ridges in the ventrolateral part of the aorta. The border of the somato- and splanchnopleures close to the aorta showed a very intensive cell proliferation and a cell emigration up to the aorta. This cell flow and the bilateral appearance of the intraaortic ridges suggested that the aortic cell clusters originated from the coelomic epithelium. This intraembryonic hemopoietic stem cell formation from the splanchnopleure was comparable to that of the blood island formation in the yolk sac from extraembryonic splanchnopleure. The appearance of the white blood cells and definitive erythrocytes with adult-type hemoglobin was preceded by the aortic cell clusters. We concluded that the stem cells of the adult-type blood developed from the aortic cell clusters whereas the blood islands of the yolk sac may contribute only the primitive red blood cells.