Which is not a constituent of juxtaglomerular complex:September 2011
Ans. B: GlomerulusThe JGA is composed of JG cells, macula densa cells and mesangial/Lacis cellsJuxtaglomerular apparatusThere are 3 different types of cells in the Juxtaglomerular Apparatus: Granular Cells/ juxtaglomerular cells, Macula Densa Cells and Mesangial Cells.Granular Cells/ juxtaglomerular cellsGranular cells are modified pericytes of glomerular aerioles.They are also known as Juxtaglomerular cells.The Juxtaglomerular cells secrete renin in response to:- Betal adrenergic stimulation- Decrease in renal perfusion pressure (detected directly by the granular cells)- Decrease in NaC1 absorption in the Macula Densa (often due to a decrease in glomerular filtration rate, causing slower filtrate movement through the proximal tubule and thus more time for reabsorption. This results in a lower NaC1 concentration by the time the filtrate reaches the Macula Densa).Macula Densa CellsMacula densa cells are columnar epithelium thickening of the distal tubule.The macula densa senses sodium chloride concentration in the distal tubule of the kidney and secretes a locally active (paracrine) vasopressor which acts on the adjacent afferent aeriole to decrease glomerular filtration rate (GFR), as pa of the tubuloglomerular feedback loop.Specifically, excessive filtration at the glomerulus or inadequate sodium uptake in the proximal tubule / thick ascending loop of Henle brings fluid to the distal convoluted tubule that has an abnormally high concentration of sodium.Na/Cl cotranspoers move sodium into the cells of the macula densa.The macula densa cells do not have enough basolateral Na/K ATPases to excrete this added sodium, so the cell's osmolarity increases.Water flows into the cell to bring the osmolarity back down, causing the cell to swell.When the cell swells, a stretch-activated non-selective anion channel is opened on the basolateral surface.ATP escapes through this channel and is subsequently conveed to adenosine.Adenosine vasoconstricts the afferent aeriole Al receptors and vasodilates (to a lesser degree) efferent aerioles A2 receptors which decreases GFR.Also, adenosine inhibits renin release in JG cells A2 receptors on JG cells using Gi pathway.Also, when macula densa cells detect higher concentrations of Na and Cl they inhibit Nitric Oxide Synthetase (decreasing renin release).Mesangial cells/ Lacis cellsMesangial cells are structural cells in the glomerulus that under normal conditions serve as anchors for the glomerular capillaries.The mesangial cells within the glomerulus communicate with mesangial cells outside the glomerulus (extraglomerular mesangial cells), and it is the latter cells that form pa of the juxtaglomerular apparatus.These cells form a syncytium and are connected with glomerular mesangial cells gap junctions.They contain actin and myosin, allowing them to contract when stimulated by renal sympathetic nerves, which may provide a way for the sympathetic nervous system to modulate the actions of the juxtaglomerular apparatus.
Macula densa
Glomerulus
juxtaglomerular cells
Extraglomerular mesangial cells