Precapillary Arteriovenous Anastomoses

Abstract

PRECAPILLARY arteriovenous anastomoses are a normal part of the microcirculatory bed of skin, muscle, lung, heart, intestine, liver, spleen, kidney, ear, and eye.1,2 These have been labeled "preferential channels"3 or "throughfare channels" by Chambers and Zweifach.4 Whether similar anastomoses occur in the brain parenchyma has been a subject of debate. Rowbotham and Little5-9 observed precapillary arteriovenous anastomoses in the pial circulatory system but not in the brain parenchyma of man, while Pfeifer10,11 and Solnitsky12 saw them in both.Benda and Brownell13 and Swank and Hain14 injected particles up to 60\g=m\ in diameter into the carotid arteries of dogs and observed that they passed to the cerebral veins. Because the average capillary diameter has been found to be 5.8\g=m\ by Ueda,15 the passage of large emboli from artery to vein means that there is a shunt bypassing the capillaries or that the capillaries have a tremendous capacity for dilatation. Despite this it has been the conclusion of the majority of studies of the angioarchitecture of the brain in both human beings and experimental animals that, with the exception of the leptomeningeal circulation, there are normally no arteriovenous anastomoses in the cerebral vascular system. Because of this disagreement, we decided to reinvestigate the subject. Material and MethodsTwelve normal human brains with the pa¬ tients ranging in age at death from 6 months to 96 years and five normal dog brains were selected. After fixation for four weeks in 15% neutral formaldehyde (formalin) solution, fro¬ zen sections 200µ to 500µ, in thickness were made from cerebrum (frontal, parietal, tempo¬ ral, and occipital lobes) and cerebellum, and the vascular system was stained selectively by a metallic impregnation method developed by T. Hasewaga and J. R. Ravens.16 One dog was injected slowly with approxi¬ mately 10 ml/kg of 14% trypan blue human plasma solution into the right femoral vein.The dog was killed, and the brain was fixed for seven days in 15% neutral formalin solution, after which one hemisphere was cut coronally and the other sagittally into blocks 2 to 3 mm thick. From these blocks frozen sections 200µ to , µ in thickness were made, employing dehydration, clearing, and mounting.In addition, frozen sections µ, to 150µ thick were made and stained with eosin picric acid solution (100 ml of alcohol eosin solution and four drops of picric acid) for 10 to 15 sec¬ onds. Dehydration, clearing, and mounting were employed immediately after staining. Arterioles were stained violet-blue and venules, light violet or pink.Another dog was infused with 14% trypan blue human plasma solution; and after it was killed, 60 ml of a mixture of india ink in 4% gelatin solution was injected into both common carotid arteries. The brain was removed and fixed in 95% alcohol for seven days, and one hemisphere was cut coronally, the other sagit¬ tally into 2 to 3 mm thicknesses. These were placed in 95% alcohol for seven days, in ab¬ solute alcohol for three days, and immersed in a so...

Anatomical Analysis of the Developing Cerebral Vasculature in Premature Neonates: Absence of Precapillary Arteriole-to-Venous Shunts

“…Although both precapillary arteriolar-to-venous (5, 27) and arteriolar-to-arteriolar shunts (27,31) have been reported in brains of premature babies, our investigation failed to identify these anatomical structures in all specimens except one. We conclude that fetal brains exhibit a circulatory pattern that …”

“…There is little evidence to support the view that brain arterioles and veins are connected by shunts. The primary source of support for arteriolar-to-venous shunts is Hasegawa et al (27), who reported finding large numbers of shunts, or "thoroughfare channels," in thick slices of both dog and human brain tissue stained to reveal arterioles, capillaries, and veins. However, Saunders and Bell (28), Duvernoy et al (17), and Moody et al (12) suggest that these shunts were misdiagnosed and that the structures reported as shunts were actually crossing vessels rather than joining vessels.…”

Pial Microcirculation in Subarachnoid Hemorrhage

“…Anatomical arteriovenous shunts below the cortical surface, 64 if present, also may shift considerable volumes of blood in the face of surface microvascular spasm. The overall effect of physiological and anatomical shunts resulting from hemorrhagic or traumatic microvascular spasm would be for blood to be flowing around, through, and past an area of cerebrum, bypassing the capillary beds, and precluding tissue exchange of metabolites.…”

Regional Cerebral Blood Flow Determination by the Hydrogen Clearance Technique and Comparison With Oxygen Availability in the Rabbit

“…The existence of precapillary sphincters is generally accepted, and there is experimental anatomical evidence for the presence of small (15 to 25/x) arteriovenous thoroughfare channels, 15 -10 which might account for some of the rapid flow rates obtained, particularly those from three compartment curves. These rapid flow rates also might be because an electrode was near an artery (particularly with the large electrodes).…”