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The Nephron: Juxtaglomerular Apparatus: figure

The juxtaglomerular apparatus (JGA), located in the renal cortex, is a unique segment of the nephron where the thick ascending limb of the loop of Henle passes between the afferent and efferent arterioles of its own glomerulus. The macula densa is the specialized area of the thick ascending limb which makes contact with the vascular elements of the JGA. The vascular elements of the JGA contain modified smooth muscle cells of the arterioles (granular cells) which contain secretory granules that synthesize and secrete the enzyme renin. A second group of JGA cells, called the lacis cells or extraglomerular mesangial cells, are not granular but also secrete renin.

The JGA plays a major role in the renin-angiotensin-aldosterone system. One theory holds that the macula densa senses changes in the Na+ or Cl? concentration and changes the rate of renin secretion. A second theory states that renin secretion is controlled by changes in volume and stretch of the afferent arteriole. The JGA has also been implicated in the autoregulation of GFR. It has been proposed that changes in Na+ or Cl? transport by the macula densa causes release of an unidentified vasoconstrictor which acts locally to control filtration pressure and GFR.