Developmental defects in the lungs.

An increasing number of congenital abnormalities of the lung is becoming recognized during life. Nineteen such cases of particular interest are described. They have been selected from a group of 37 patients with congenital lung defects, of which 35 have been operated on by one surgeon since 1939. In five cases the intrathoracic abnormalities are threefold, in nine a double abnormality is present, while in two cases congenital abnormalities elsewhere in the body coexist with the pulmonary malformation. Cases of purely vascular abnormality without pulmonary defect, for example, arteriovenous fistula, aneurysm, or pulmonary artery atresia, have not been included. Brief reference to the embryology is necessary. The tracheobronchial diverticulum from the floor of the foregut appears at the 3 mm. stage, and gradually separates off from the rapidly lengthening oesophageal rudiment. Caudal bifurcation with condensation of splanchnopleuric mesoderm gives rise to the primitive lung bud and stem bronchus in the 4 mm. embryo. Further growth and branching of the stem bronchi continues until some 18 generations of subdivision have occurred by the time of birth (Broman, 1923). During these changes the intra-embryonic coelom becomes modified to form a large pericardial cavity bounded caudally by the septum transversum through which the vitelline and umbilical veins and the ducts of Cuvier pass to the sinus venosus. This pericardial cavity communicates freely on either side of the septum transversum with the peritoneal cavity by way of the pericardio-peritoneal canals. It is into these canals that the developing lung buds project so that each primitive pleural cavity now possesses a wide communication with the pericardial and peritoneal cavities. Increasing growth of the lung leads to differential enlargement of the pleural cavity which passes behind and lateral to the duct of Cuvier, so producing the pericardio-pleural fold. Closure of the pleuroperitoneal canal results from proliferation of tissue in the free edge of this fold combined with increasing obliquity of the duct of Cuvier due to caudal migration of the heart. It occurs first on the right side, and is usually complete by the 10 mm. stage. The portion of the primitive vascular plexus in relation to the foregut diverticulum develops further in the mesoderm of the lung buds. Descending branches from the sixth pharyngeal arch arteries reach the plexus to become the pulmonary arteries by the 10 mm. stage. At this stage the bulbus cordis has become subdivided by growth of the spiral aorticobulbar septum so that blood from the primitive right ventricle reaches the lungs via the dorsal portion of the aortic sac. The oesophageal and bronchial arteries are persistent splanchnic segmental arteries cranial to the coeliac axis, and the small arteries in the pulmonary ligament are probably similarly derived.