Developmental biology of the pulmonary circulation.

From the earliest stage of lung development, there is an accompanying blood circulation. In the adult lung, the pulmonary arteries are closely associated with the airways. During early fetal development, the airways act as a template for pulmonary blood vessel development in that the vessels form by vasculogenesis around the branching airways. In later lung development, as the alveoli multiply, new capillaries form by angiogenesis. As blood vessels increase in size, they develop a muscle wall that is relatively thick during fetal life and shows a rapid reduction after birth. The control of development by growth factors and the physiological changes immediately after birth are described in this review. Abnormal pulmonary vascular development leading to pulmonary arterial hypertension and strategies for treatment are also discussed.