Juxtaglomerular apparatus

Juxtaglomerular apparatus or (juxtaglomerular complex) as indicated by the name (juxta — near) refers to the collection of specialized cells located between the distal convoluted tubule and the afferent arteriole.

Function of Juxtaglomerular apparatus

The primary function of the juxtaglomerular apparatus is to secrete the hormones. It also regulates the glomerular blood flow and the (glomerular filtration rate).

Hormone secretion secretes two hormones:

1. Renin: juxtaglomerular cells of juxtaglomerular apparatus secrete renin. Renin is a peptide containing 340 amino acids. Along with angiotensin, renin forms the renin-angiotensin system, a hormone system that plays an important role in maintaining blood pressure.
2. Prostaglandin: Extraglomerular mesangial cells of juxtaglomerular apparatus secrete prostaglandin. Prostaglandin is also secreted by interstitial cells of medulla called type I medullary interstitial cells.

1. Macula densa cells

Macula densa is the final component of the thick ascending segment before it opens into a distal convoluted tubule. It is found between the afferent and the efferent arterioles of the same nephron in the wall of the (distal convoluted tubule). It is very close to the afferent arteriole. The macula densa is formed by tightly packed cuboid epithelial cells.

2. Juxtaglomerular cells

Juxtaglomerular (JG) cells are specialized myoepithelial (modified vascular smooth muscle) cells found in the wall of the afferent arteriole just before entering the Bowman capsule. These smooth muscle cells are mainly present in the tunica media and the tunica adventitia wall of the afferent arteriole. Juxtaglomerular cells are also called granular cells because of the presence of secretory granules in their cytoplasm.

3. Extraglomerular mesangial cells

Extraglomerular mesangial cells are located in the triangular region bound by afferent arteriole, efferent arteriole, and macula densa. These cells are also referred to as agranular cells, lacis cells, or Goormaghtigh cells.

Glomerular Mesangial Cells

In addition to extraglomerular mesangial cells, there is another type of mesangial cells located between glomerular capillaries called mesangial glomerular or intraglomerular mesangial cells. Glomerular mesangial cells support glomerular capillary loops by surrounding capillary cells in the form of a cellular network.

These cells play an important role in regulating glomerular filtration by their contractile properties. Glomerular mesangial cells are phagocytic. These cells also secrete glomerular interstitial matrix, prostaglandins, and cytokines.

Characteristic features of these cells are:

• Both the macula densa cells (on one side) and the juxtaglomerular cells (on the other side) are in contact.
• Structurally, these cells serve as supporting cells and plugging devices at the glomerular entry.
• Functionally, these cells can relay signals from the macula densa to the granular cells after the signals have been modulated. Decreased intraluminal Na+ load, Cl−load or both in the macula densa region stimulates the juxtaglomerular cells to secrete renin.
• They also demonstrate granulation to secrete renin under conditions of severe hyperactivity.
• They also secrete various substances and take up immune complexes.