THE JUXTAGLOMERULAR APPARATUS

Juxtaglomerular apparatus is a specialized organ situ­ated near the glomerulus of each nephron. The JGA is made up of three different structures: 1. Macula densa 2. Extraglomerular mesangial cells 3.

Macula densa is the end portion of the thick ascending segment before it opens into the distal convoluted tubule. It is situated between the afferent and efferent arterioles of the same nephron and is very close to the afferent arteri­ole. The macula densa is formed by tightly packed cuboidal epithelial cells. It helps in regulating the glomerular filtra­tion rate, renal blood flow, and renin release. Macula densa cells detect intratubular salt concentrations to control renal blood flow through afferent arteriole constriction and dila­tion (Figure 18.5).

Extraglomerular mesangial cells are situated in the trian­gular region surrounded by the afferent and efferent arteri­oles and macula densa. These cells are also called agranular cells or Lacis cells. They resemble smooth muscle cells and play a role in renal autoregulation of blood flow to the kid­ney and the regulation of systemic blood pressure through RAS.

1. Juxtaglomerular granular cells: The smooth muscle cells of the afferent arteriole that make contact with the macula densa are specialized smooth muscle cells and are called juxtaglomerular (JG) granular cells. Juxtaglomerular cells are also called granular cells because of the presence of secretory granules in their cytoplasm that contain renin, a proteolytic enzyme.

2.Glomerular mesangial cells: The space between the macula densa and the afferent and efferent arterioles, as well as the space between the glomerular capillaries, is known as the mesangial region and consists of mesangial cells. Mesangial cells secrete the matrix, glomerular base­ment membrane, provide structural support, have phago­cytic activity, secrete prostaglandins, and also exhibit contractile activity and can influence blood flow through glomerular capillaries.

The secretion of hormones is the primary function of the juxtaglomerular apparatus. It also regulates the glomeru­lar blood flow and glomerular filtration rate. The juxtaglo­merular apparatus secretes two hormones, i.e., renin and prostaglandin. Renin plays an important role in the mainte­nance of blood pressure. Secretion of renin is stimulated by four factors: i. A fall in arterial blood pressure ii. A reduc­tion in the ECF volume iii. Increased sympathetic activity iv. A decreased load of sodium and chloride in the macula densa. Once renin is secreted, it converts angiotensinogen to angiotensin I. Angiotensin I is converted to angiotensin II by another enzyme called angiotensin-converting enzyme (ACE). Angiotensin II acts as a potent vasoconstrictor and helps in the restoration of decreased blood volume.

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