Aglomerular pathways to the medulla

By Supriyadi Pro -
GLOMERULAR SHUNTS - A number of investigators have described glomeruli in which the afferent and efferent arterioles are continuous across the vascular pole, the glomerular capillaries branching off from the afferent arteriole or the connecting vessel.

These afferent-efferent shunts occur almost exclusively in the juxtamedullary glomeruli. Ljungqvist (1964), in a survey of 180 normal and diseased human kidneys, showed by angiography combined with histological methods that, while in the cortical zone the glomeruli showed the normally accepted pattern in which the afferent arteriole, capillary tuft and efferent arteriole are arranged in series, in most of the juxtamedullary glomeruli there was direct continuity between afferent and efferent vessels.

In fetal and neonatal kidneys, however, the juxtamedullary glomeruli showed the cortical pattern. It was suggested that the afferent-efferent shunt in the juxtamedullary glomeruli might explain the differing reaction of glomeruli to disease. Thus, during glomerular degeneration, for example in the hypertensive kidney, in the cortical region there is atrophy of the glomerulus along with both its vessels whereas in the juxtamedullary region there is an increase in size of the shunt so that the medullary blood supply is actually improved.

Ljungqvist’s work has largely been confirmed by Takazakura et al. (1972) who found that in sixty-three human kidneys, the frequency with which continuity between afferent and efferent arterioles could be demonstrated increased with age although even at the age of 9 months, 9 per cent of juxtamedullary glomeruli showed continuity.

After the age of 45, the majority of vessels were continuous. In diseased kidneys, afferent-efferent shunts were very much more common than would be expected from a consideration of age alone and in glomerulonephritis over 90 per cent of glomeruli were affected in this way, regardless of age.

Ljungqvist (1975) has recently studied the fine structure of the shunt in ten rat glomeruli. He found that the afferent and efferent arteriole are connected by a vessel segment consisting of an endothelium surrounded by cells which resemble the lacis cells of the juxtaglomerular apparatus. The glomerular capillaries arise from the glomerular side of the segment while the opposite side is in close contact with the macula densa of a distal tubule.

AGLOMERULAR VASA RECTA
With increasing age and in certain diseases, particularly hypertension, there is an even wider communication between afferent and efferent arterioles in the juxtamedullary region. The glomerular capillary tuft becomes atrophiq and the complex vascular pathway through the glomerulus becomes reduced to a smaller number of wider vessels until there is complete continuity, the remains of the capillaries being represented only by a scarcely recognizable remnant.

Eventually, the vasa recta appear to be arising directly from a branch of the arcuate or interlobular arteries---the so-called ‘vasa recta vera’ or (preferably) ‘aglomerular vasa recta’ (Fig. 1.8/Read: The vasa recta). Ljungqvist (1962) found that in severe renal damage due to hypertension the greater part of the medullary blood supply is by means of aglomerular vessels.

Whether these vessels should be regarded as pathological in all cases or as the result of a normal developmental process is not clear, since Ljungqvist (1963) found such vessels at the age of 7 years and I have seen them even earlier. Ljungqvist and Lagergren (1962) studied the aglomerular vessels of the human kidney by angiography and in histological preparations and found remnants of glomeruli attached to most of the apparently aglomerular vessels.

Ever since Virchow’s day the aglomerular vasa recta have been known as vasa recta vera’ while the normal vasa recta derived from glomerular efferent arterioles have been called ‘vasa recta spuria', Indeed, they have often been ‘arteriolae’ rather than ‘vasa’.

These terms should now be dropped-‘arteriolae’ on the grounds that no smooth muscle is found in the walls of the vessels except at their commencement and the other terms because the ‘vasa recta spuria’ are the true vasa recta while the ‘Vasa recta vera’ are the result of secondary, often pathological, changes.

Read the full article: The Anatomy of the Renal Circulation

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