Development of the right outflow tract and pulmonary arterial supply.

The branchial arch vessels of the human embryo have been studied by histological and radiographic methods and the modelling that occurs during the period Day 25-Day 52 postfertilization is described. It has been shown that the myoendocardial reticulum is reamed out by blood flow and it is suggested that hydrodynamic force is the fundamental factor which determines chamber structure of the heart and flow pattern in the outflow tracts and great vessels. The sixth aortic arch vessels contribute tissue to the pulmonary trunk and proximal pulmonary arteries. The 'postbranchial pulmonary arteries' are morphologically distinct and form the pulmonary arteries at the lung hila. The primitive pulmonary plexus around the tips of the developing tracheobronchial primordia is formed from segmental vessels arising from the dorsal aorta. Bronchial arteries can be demonstrated only late in intrauterine life. The numerous bronchopulmonary precapillary anastomoses which are found in the fetus at this time have been demonstrated radiographically.