Development of Blood Vessels

Blood vessel development starts outside the embryo on day 15 or 16 in the mesoderm of the yolk sac, chorion, and connecting stalk. Approximately two days later, blood vessels begin forming inside the embryo itself. This early vascular formation is facilitated by the small amount of yolk present in the ovum and yolk sac.

Hemangioblasts are vascular precursor cells that give rise to angioblasts, which are responsible for producing blood vessels. These angioblasts aggregate to form structures called blood islands throughout the embryonic discs. Spaces within these islands become the lumens, or inner open spaces, of developing blood vessels.

Blood islands develop cavities that become the lumens of blood vessels. Angioblasts surrounding these cavities differentiate into the endothelial lining of blood vessels, while others form the tunics of larger vessels. Blood islands continue to expand and merge, creating an extensive network of blood vessels throughout the developing embryo.

The local concentration of vascular endothelial growth factor in the surrounding area determines whether a blood vessel will develop into an artery or vein. This differentiation factor acts as a molecular signal that guides vessel specialization during embryonic development, establishing the distinct characteristics of arterial and venous structures.

By the end of the fourth week of development, the rudimentary fetal heart is capable of pumping blood through the developing vascular system. This establishes a functional connection between the embryo and the placenta, enabling the circulation of blood through the embryonic vascular network that has formed during the preceding weeks.

Many blood vessels follow the same guidance cues as nerves during embryonic development, often nestling closely to them. This coordinated development ensures that the vascular system and nervous system develop in spatial proximity, supporting efficient nutrient delivery and neural function throughout the developing embryo.

The embryonic vascular network formed during early development establishes the foundation for fetal circulation. By week four, the heart pumps blood through this network, creating the circulatory pathways necessary for fetal development and establishing connections to the placenta for gas and nutrient exchange.