Juxtaglomerular apparatus • Located at the vascular pole of Bowman’s capsule • Formed by conjunction of cells of (1) macula densa (distal straight tubule) (2) juxtaglomerular cells of the afferent arteriole (3) extraglomerularmesangial cells (Lacis cell) • Involved in the regulation of systemic blood pressure via the renin-angiotensin-aldosterone mechanism and autoregulation of kidney functions. Macula densa – (1) The macula densa is an area of closely packed, specialized DCT epithelial cells where the DCT abuts the vascular pole of the glomerulus. (2) The cells of the macula densa are taller and have larger more prominent nuclei situated towards the luminal surface. (3) The cells of the macula densamonitor the Na concentration in the tubular fluid and regulate both the glomerular filtration rate and the release of renin from the juxtaglomerular cells. They are sensitive to the concentration of sodium ions in the fluid within the DCT; a decrease in systemic blood pressure results in decreased production of glomerular filtrate and hence decreased concentration of sodium ions in the distal tubular fluid. Thus macula densa acts as chemoreceptors (osmoreceptors). (4) If sodium concentration is below a specific threshold, macula densa cells do two things: 1. They cause dilation of the afferent glomerular arterioles through Lacis cells and intraglomerular mesangial cells, thus increasing blood flow into the glomerulus. 2. They instruct juxtaglomerular cells to release the enzyme renin into circulation. The renin then constricts efferent arteriole via angiotensin II. (5) Both the effects ultimately increase glomerular capillary hydrostatic pressure and ultimate glomerular filtration. Juxtaglomerular cells – (1) Specialized smooth muscle cells of the wall of afferent arteriole just before it enters the glomerulus, which contain renin in secretory granules. (2) The juxtaglomerular cells are baroreceptors that are sensitive to changes in blood pressure. When blood pressure drops, these cells release renin which sets into play a series of reactions that lead to vasoconstriction and absorption of water from the filtrate into the blood. Extraglomerularmesangial cells –Goormaghtigh cells or lacis cells (1) These cells form a conical mass, the apex of which is continuous with the mesangium of the glomerulus; laterally it is bounded by the afferent and efferent arterioles and its base abuts the macula densa. These cells form a syncytium and are connected with glomerular mesangial cells via gap junctions. (2) The current theory is that these cells participate in the tubuloglomerular feedback mechanism by which changes in Na+ concentration at the macula densa give rise to signals that directly control glomerular blood flow. The extraglomerular mesangial cells are thought to be responsible for transmission of a signal arising in the macula densa to the intraglomerular mesangial cells which then contract or relax to make the capillary loops narrower or wider.
Posted on: Thu, 28 Aug 2014 03:27:08 +0000