The Juxtaglomerular Apparatus

Reference was made earlier to the macula densa, a portion of the late thick ascending limb at the point where, in all nephrons, this segment comes between the afferent and efferent arterioles at the vascular pole of the renal corpuscle from which the tubule arose. This entire area is known as the juxtaglomerular (JG) apparatus. (Do not confuse the term juxtaglomerular apparatus with juxtamedullary nephron.) Each JG apparatus is made up of 3 cell types: (1) granular cells, also called juxtaglomerular cells, which are differentiated smooth muscle cells in the walls of the afferent arterioles; (2) extraglomerular mesangial cells; and (3) macula densa cells, which are specialized thick ascending limb epithelial cells.
The granular cells (so called because they contain secretory vesicles that appear granular in light micrographs) are the cells that secrete the hormone renin. Renin is a crucial substance for the control of renal function and systemic blood pressure. The extraglomerular mesangial cells are morphologically similar to and continuous with the glomerular mesangial cells but lie outside Bowman’s capsule. The macula densa cells are detectors of the luminal content of the nephron at the very end of the thick ascending limb and contribute to the control of glomerular filtration rate (GFR) and to the control of renin secretion.