Macula densa

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In the kidney, the macula densa is an area of closely packed specialized cells lining the region of the distal convoluted tubule (DCT) lying next to the glomerular vascular pole.

The cells of the macula densa are sensitive to the ionic content and water volume of the fluid in the DCT, producing molecular signals that promote renin secretion by other cells of the juxtaglomerular apparatus.^[1] The release of renin is an essential component of the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and volume.

1 Histology
2 Function
3 References
4 External links

[edit] Histology

The cells of the macula densa cells are taller and have more prominent nuclei than surrounding cells of the distal convoluted tubule.

The close proximity and prominence of the nuclei cause this segment of the DCT's wall to appear darker in microscopic preparations^[2], hence the name macula densa.

[edit] Function

Schematic depicting how the RAAS works. Here, activation of the RAAS is initiated by a low perfusion pressure in the juxtaglomerular apparatus

A decrease in blood pressure results in a decreased concentration of sodium ions in the distal convoluted tubule. (This is due to reduced filtration by the glomerulus: less filtrate is expelled into Bowman's space and the proximal convoluted tubule; the resulting fluid reaching the distal convoluted tubule will have a lower sodium concentration after the water is removed.)

In response, the macula densa cells release prostaglandins, which triggers granular juxtaglomerular cells lining the afferent arterioles to release renin into the bloodstream. (The juxtoglomerular cells can also release renin independently of the macula densa, as they are also triggered by baroreceptors lining the arterioles, and release renin if a fall in blood pressure in the arterioles is detected.)