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It is lined by a single layer of cuboidal to low columnar cells with rounded nuclei and eosinophilic granular cytoplasm. Cell boundaries interdigitate with those of adjacent cells laterally and basally. The proximal convoluted tubule also possesses an apical brush border that provides the cell with a much greater surface area for reabsorption from, and secretion into, the fluid that becomes urine in the kidney tubules. In fact, most of the components of the glomerular filtrate are reabsor-bed in the proximal tubule. Loop of Henle is a hairpin loop of the nephron that extends into the medulla and consists of thick and thin segments. The thick proximal portion of Henle's loop, or the descend ing thick segment, is a direct medullary continuation of the cortical proximal convoluted tubule.

The descending and ascending thin segments of the loop of Henle are lined by a single layer of flat epithelial cells with nuclei that bulge into the lumen.

The thick distal portion of the loop of Henle, the ascending thick segment, ascends to the cortex and is continuous with distal convoluted tubule. It is lined by cuboidal cell; that contain numerous invagina-tions of cytoplasm anc many mitochondria. Distal convoluted tubule is lined by cuboidal cells that contair a granular cytoplasm. Cells of the distal convoluted tubule near the afferent arteriole are taller and more slender than elsewhere in the distal tubule. They constitute the macula densa. Their nuclei are packed closely, so the region appear darker under the light microscope. The macula densa is thought to sense sodium concentration in the tubular fluid. The major function of the distal tubule is to reabsorb soduim and chloride from the tubular filtrate. Collecting tubules consist of arched and straight segments. The arched collecting tubule segments are located in the cortical labyrinths and empty into the straight collecting tubule segments, which pass through the medullary rays. Epithelial cells of the collecting ducts range from cuboidal to columnar. Identification of these tubules is facilitated by their distinct intercellular borders as a result of the lack of complex in-terdigitations seen in the proximal and distal tubules. Vascular supply begins with the renal artery, enters the kidney the hilum, and immediately divides into interlobar arteries. The arteries supply the pelvis and capsule before passing direct between the medullary pyramids to the corticomedullary junction. The interlobar arteries bend almost 90 degrees to form shoarching, arcuate arteries, which run along the cortico-medullary junction. The arcuate arteries subdivide into numerous fine interlobul arteries, which ascend perpendicularly to the arcuate arteries through the cortical labyrinths to the surface of the kidney. Each in-terlobular artery passes midway between two adjacent medullary rays.

The interlobular arteries then give off branches that become the afferent arterioles of the glomeruli.

As the afferent arteriole approaches the glomerulus, some its smooth muscle cells are replaced by myoepithelioid cells, which are part of the juxtaglomerular apparatus. The juxtaglomerular apparatus consists of juxtaglomerular cells, polkissen cells, and the macula densa.