The abnormal pulmonary venous connexion: a developmental approach

If embryonic communications exist between the splanchnic and cardinal venous systems, then they can explain variations in partially or completely abnormal pulmonary venous return as seen in congenitally malformed hearts. With this in mind, we investigated complete serial sections of 52 human embryos, ranging from 4.4 to 25 mm crown-rump length (Streeter's horizons XIII-XXII) and 57 mouse embryos (Mus musculus albus CPB-S) from 7.8 to 19.7 days of gestation. Specifically, we investigated whether communications persisted between the splanchnic, the pulmonary and the cardinal venous systems. In the early stages of development, the pulmonary vascular bed shared the route of drainage of the peripheral splanchnic plexus into the umbilical-vitelline and cardinal venous systems. As the pulmonary plexus developed, it obtained a new direct route of drainage into the left atrium via the pulmonary veins. A persistence of embryonic connexions was encountered in one human and 14 mouse embryos. In all these cases, the stage of development was such that the connexions between the splanchnic and pulmonary plexuses should have disappeared. An abnormality in the development of the central pulmonary venous system was also found in the human embryo. Whether this was primary or secondary to the persistence of the embryonic connexions remained unresolved.