Macula densa apparatus

Cyclooxygenase (COX) activity is responsible for the formation of prostaglandins from their arachidonic acid precursor. Two COX isoforms have been identified, COX-1 and COX-2. While COX-1 is constitutively expressed in most tissues, COX-2 is typically only found after induction by proinflammatory stimuli. However, a constitutively expressed and highly regulated COX-2 is found in the kidney, both in the renal medulla and in the renal cortex. Renal cortical COX-2 is located in the area ofthe juxtaglomerular apparatus, and prostaglandins formed by COX-2 regulate the expression and secretion of renin in response to a reduction in NaCl concentration at the macula densa. 

A correlation between the activity of G-6-PDH and extractable renin in rat kidneys has been reported, suggesting that the macula densa cells and also the juxtaglomerular apparatus are parts of a system related to the formation of renin (H6). 

Thurau K, Schnermann J, Nagel W, Horster M, Wahl M. Composition of tubular fluid in the macula densa segment as a factor regulationg the function ofthejuxtaglomerular apparatus. Circ Res 1967 20-21 (SuppI 2) 79-90. 

The basement membrane has collagen and glycoproteins as its major constituents. Anionic regions present on the basement membrane influence the filtration of molecules. The juxtaglomerular apparatus located at the hilus of the glomerulus consists of three parts. These are the macula densa, which is a small segment of the distal tubule near the afferent arteriole, the epithelioid cells, which are the site of renin synthesis, and the lacis cells located in the region between the macula densa and the afferent and efferent arterioles (Fig. 3). 

Fig. 317. Juxtaglomerular apparatus with Goormaghtigh cells (partly split up into their processes) and macula densa from the kidney (block BNh 3372) of a male Wistar rat (No. 2567) 90 min after a single intraperitoneal injection of 5 mg N-benzhydryl-N -p-hydroxybenzylpiperazine HCl per kg body weight. Under pentobarbital anaesthesia (30 mg/kg), the animal was perfused from the abdominal aorta with 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH 7.4). Postfixation with 1 % osmium tetroxide in sodium cacodylate buffer. Embedded in Epon 812 and sectioned at 50 nm. Lead citrate and uranyl acetate. Plate 2368...

Several in vitro studies in isolated juxtaglomerular cells showed that NO either inhibited (Kurtz et al. 1986, Henrich et al. 1988, Greenberg et al. 1995) or stimulated (Gardes etal. 1992, Scholz and Kurtz 1993) renin secretion. Studies on the isolated perfused juxtaglomerular apparatus indicated that macula densa-generated NO is impor-... 

Fig. 9-7. Hyperplastic juxtaglomerular apparatus in renovascular hypertension. The juxtaglomerular cell mass (polar cushion) is at the hilus of the glomerulus, next to the macula densa and between the afferent arteriole (which enters the glomerulus here on the right side) and the efferent arteriole (which is not in the plane of this section). Instead of its normal 6 to 8 cells, this polar cushion contains over 30 cells. Hematoxylin and eosin stain, x 520. (Supplied by Harrison Latta)...

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