Blood vessels and circulation

Blood cells and blood vessels

Blood vessels develop when the embryo reaches such a size that diffusion no longer supplies sufficient nutrition and oxygen. At S6, blood islands begin to develop from mesenchymal cells in the wall of the umbilical vesicle. The marginal cells of these blood islets become angioblasts, which further develop to become endothelial cells and form the wall of a vesicle. The central cells become haemocytoblasts (haematopoietic stem cells) from which different types of blood cells differentiate. Adjacent vesicles fuse together and form a network of blood vessels outside the embryo. From S9, intra-embryonic blood vessels start to arise within the embryonic body independently of these extra-embryonic blood vessels, especially in the visceral mesoderm. The two networks of blood vessels grow towards each other and become interconnected.