Endothelial vessels can be formed in two different ways before blood circulation starts. The first way is through angioblastic vessel formation (or vasculogenesis), whereby new vessels arise in situ; for example, the endothelial lining of the heart, the dorsal aortae, the umbilical vessels and the early omphalomesenteric vessels. The second way is through angiotrophic vessel formation (or angiogenesis), by which vessels arise through sprouting and ramification from the endothelium of existing vessels.
Major blood vessels usually develop from a capillary plexus. Later, the endothelial tubes are surrounded by connective tissue and smooth muscle cells, giving rise to arteries and veins. The pattern of vessel formation depends on both local and haemodynamic factors.