Development of the circulatory system.

The blood vessels and heart first appear as collection of endothelial cells that differentiate from mesenchymal cells. The heart, main blood vessels, and peripheral vessels develop independently and later unite to establish a blood circulation.

Arteries and veins of all types first appear as ordinary capillaries, which later increase in size and acquire smooth muscle and connective tissue by differentiation from the surronding mesenchyme. After the establishment of the circulation, new vessels arise by budding from pre-existing vessels.

The heart in the earliest human embryos is a tube with a double wall: the internal endothelium, from which the endocardium develops, and the external myoepicardial layer. From the myoepicardial layer are formed both myocardium and epicardium. The endocardium has important roles in the formation of partitions to separate the primary single cavity of the heart into chambers, and in the formation of the valves.

There are two opinions concerning the mode of development of the lymphatic vessels. According to one view the lymphatic vessels arise as evaginations or buds from veins. Most observers, however, believe that the lymphatics first arise independently from the veins as isolated intercellular clefts in the mesenchyme. The mesenchymal cells lining the clefts differentiate into endothelial cells, and the clefts enlarge and fuse to form the primary lymphatic vessels. These later establish communication with the veins. Later development of the lymphatic stem occurs mainly by budding from the wall of pre-existing lymphatics in much the same way as in the blood vessels.

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