- It is seen that a part of the distal convoluted tubule (at the junction of its straight and convoluted parts) lies close to the vascular pole of the renal corpuscles that is between the afferent and efferent arterioles.
- The muscle cells in the wall of the afferent arterioles are modified in this region.
- They are large and rounded with spherical nuclei.
- Granules are present in their cytoplasm that can be stained with special methods. These are juxtaglomerular cells.
- Unmyelinated adrenergic nerve fibres innervate these cells.
- Juxta medullary cells are considered as highly modified myoepithelial cells which contain contractile filaments in the cytoplasm.
- At the site of contact with the arteriole, the wall of the distal convoluted tubule is also modified.
- The cells lining it are densely packed together here and are columnar (rather than cuboidal as in the rest of the tubule). These cells form the macula densa.
- Juxtaglomerular cells and the cells of macula densa are in close contact with each other and togetherly, they are called juxtaglomerular apparatus.
Image Source: Science Direct.
- By EM, the granules of the juxtaglomerular cells are seen to be membrane bound secretory granules.
- They contain an enzyme called renin which acts on a substance called angiotensinogen present in blood and converts it into angiotensin I.
- Similarly, there is another enzyme in the lungs that converts angiotensin I into angiotensin II.
- Angiotensin II increases blood pressure and also stimulates the secretion of aldosterone by the adrenal cortex, thus influencing the reabsorption of sodium ions by the distal convoluted tubules and that of water though the collecting ducts.
- Juxtaglomerular apparatus has also a third component in addition to the renin producing cells and the macula densa, called lacis cells which function is unknown.
- These cells are so called as they bear processes that form a lace-like network which are located in the interval between the macula densa and the afferent and efferent arterioles.
- The degree of resorption of ions by the renal tubule is controlled by a mechanism, called juxtaglomerular apparatus.
- The cells of the macula densa appear likely to monitor the ionic constitution of the fluid passing across them (within the tubule).
- The release of renin by juxtaglomerular cells appears to be influenced by cells of the macula densa.
- The function of renin is to influence aldosterone production through angiotensin II and controls tubular resorption.
- In this way, regulation of plasma volume and blood pressure occurs.
- In addition to renin the kidney produces the hormone erythropoietin which stimulates the erythrocyte production.
- It is claimed that erythropoietin is produced by juxtaglomerular cells but the site of production of the hormone is uncertain.