Macula densa

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The macula densa is a collection of densely staining epithelial cells found in the distal straight tubule of the nephron as it makes contact with the arterioles at the vascular pole of its related renal corpuscle. Collectively with the adjacent juxtaglomerular and extraglomerular mesangial cells, the macula densa forms the juxtaglomerular complex, which regulates the function of each nephron.

Under light microscopy, the cells of the macula densa are readily identifiable due to their slender, columnar morphology which gives them a ‘packed’ appearance relative to the adjacent cells of the distal tubule, which are more cuboidal. Thus the reason for the name “macula densa” (lit. ‘dense spot’).

The macula densa is primarily involved in the regulation of glomerular filtration rate in the nephron. Elevated arterial blood pressure increases the glomerular filtration rate, which leads to higher Na⁺/Cl^- concentrations in the tubular filtrate of the nephron. Such increases of ion levels are detected by the macula densa, which subsequently releases various compounds that induce vasoconstriction of the afferent glomerular arteriole; this reduces blood flow to the glomerulus causing a reduction of glomerular filtration rate and Na⁺/Cl^- concentrations in the tubular filtrate. The macula densa also influences the release of renin from the nearby juxtaglomerular cells and therefore is indirectly involved in the renin-angiotensin-aldosterone system which regulates arterial blood pressure.