Macula densa

Renal corpuscle. Macula densa is #7.

In the kidney, the macula densa is an area of closely packed specialized cells lining the wall of the distal tubule at the point of return of the nephron to the vascular pole of its parent glomerulus glomerular vascular pole.

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

Histology

The cells of the macula densa are taller and have more prominent nuclei than surrounding cells of the distal straight tubule (cortical thick ascending limb).

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

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 causes a decrease in the GFR (glomerular filtration rate) which causes more reabsorption, resulting in a decreased concentration of sodium and chloride ions at the macula densa and triggers an autoregulatory response to increase reabsorption of ions and water in order to return blood pressure to normal. Reduced blood pressure means decreased venous pressure and hence a decreased peritubular capillary pressure. This causes a smaller capillary hydrostatic pressure which causes an increased absorption of sodium ions into the vasa recta at the proximal tubule. Because of this increased absorption, less NaCl is present at the distal tubule which is where the macula densa is located. The macula densa senses this drop in salt concentration and responds through two mechanisms: first, it triggers dilation of the renal afferent arteriole, decreasing afferent arteriole resistance and thus offsetting the decrease in glomerular hydrostatic pressure caused by the drop in blood pressure. Second macula densa cells release prostaglandins, which triggers granular juxtaglomerular cells lining the afferent arterioles to release renin into the bloodstream. (The juxtaglomerular 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.) Furthermore, activation of the sympathetic nervous system stimulates renin release through activation of beta-1 receptors.

The process triggered by the Macula densa helps keep the glomerular filtration rate (GFR) fairly steady in response to varying artery pressure, due to dilation of the afferent arterioles and the action of Renin, which triggers constriction of the efferent arterioles, both of which increase hydrostatic pressure in the glomerulus.

References

External links

• Organology at UC Davis Urinary/mammal/cortex1/cortex5 - "Mammal, kidney cortex (LM, Medium)"
• Physiology at MCG 7/7ch03/7ch03p17 - "The Nephron: Juxtaglomerular Apparatus"