Day 2: The Heart of the Matter

Development of the Circulatory System in the Embryo

While the embryo is still only a few layers thick, oxygen and other nutrients can pass easily to the individual cells. But as the embryo grows, the oxygen and other nutrients can no longer reach inner cells by simple diffusion. Long before this crisis occurs, development of the circulatory system is underway. By 24 hours, pockets of blood cells and developing blood vessels, called blood islands, begin to appear on the surface of the yolk along the outer edges of the area opaca. The heart, blood vessels and blood cells are all derived from mesoderm. Around 25-26 hours the head of the embryo has begun to rise and the pocket of the foregut can be seen. In the process patches of splanchnic mesoderm, lying on either side of the head fold, closely follow the endoderm of the foregut and wrap under the pharynx to fuse and form a single straight tube. This simple straight tube is the early heart.

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