[Two cases of the retroaortic left renal vein and a morphogenetic consideration of the anomalous vein].

The left renal vein rarely passing behind the abdominal aorta is called "the retroaortic left renal vein". We encountered two cases of the retroaortic left renal vein during the student course of dissection at Iwate Medical University School of Medicine in the years 1986-1997. The incidence of the retroaortic left renal vein was calculated at 2/266 or 0.75%. We observed and recorded the two cases of the retroaortic left renal vein by photographs and line drawings. Then, to consider the morphogenesis of the anomalous vein, we studied 16 cases of the renal collar (the circumaortic renal venous ring) from 149 bodies. Moreover, we observed and recorded the relations between the left renal vein and the lumbar veins in 19 bodies dissected in 1996. Results were compared with those of the reports by some different authors and conclusions were as follows. 1. The incidence of the retroaortic left renal vein is estimated approximately at 0.75%. 2. The retroaortic left renal vein is derived from the renal collar (circumaortic renal venous ring) at an embryonic stage and is completed by the regression and disappearance of the ventral (preaortic) limb and the persistence of the dorsal (postaortic) limb at a later stage. The ventral limb originates from the anastomosis between the subcardinal veins and the dorsal limb originates from the anastomosis between the supracardinal veins (external vertebral venous plexus). The left lumbar veins drain into the inferior vena cava by using the intersupracardinal anastomosis (external vertebral venous plexus). The dorsal limb and the left lumbar veins are considered to use a same venous route passing dorsal to the abdominal aorta. Indeed, in the 16 cases of the renal collar studied, the dorsal limb use the left second lumbar vein in 7 cases, the third lumbar vein in 6 cases, the fourth lumbar vein in 1 case, both the second and the third lumbar veins in 1 case and unknown lumbar vein in 1 case. Moreover, in the two cases of the retroaortic left renal vein the dorsal limb use the third lumbar vein. 3. The retroaortic left renal vein of our cases leaves the renal hilus behind the renal artery (usually in front of the artery) at the level of the intervertebral disc between the second and the third lumbar vertebrae (just one verteral body lower than usual) and flows into the inferior vena cava by using the left third lumbar vein. The reason why ventral (normal) route of the left renal vein disappear may be that the vein leaves the renal hilus at the lower level and the more dorsal position than usual.