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For iPad and iPhone users, the links listed just below open the new virtual slides for this lab in a separate window.
Virtual Slides for Lab 7, URINARY SYSTEM:
051 Kidney PAS
053A Kidney Plastic H&E
75d2 Kidney -Monkey
77b Urinary Bladder
081B Prostate & Urethra
The objectives for the lab are the following:
Kidney: capsule, cortex and medulla, medullary rays, long and short nephrons, uriniferous tubules, renal corpsucle, glomerulus, Bowman's capsule (parietal and visceral layers), Bowman's space, endothelial cells, podocytes, mesangial cells (intra- and extraglomerular), vascular pole, macula densa, urinary pole, proximal tubule, thick and thin loops of Henle, distal tubule, collecting duct, vasa recta, juxtaglomerular complex.
The kidneys are two bean-shaped organs located retroperitoneally. The kidney is composed primarily of nephrons, there being about 1 to 1.5 million in each human kidney. The nephron is comprised of the renal corpuscle where the fluid that becomes urine is initially formed, and a tubule which modifies the composition of the fluid within it.
This is a longitudinal section of a part of a lobe of the kidney. Each kidney is divided into an outer region or cortex which contains all of the renal corpuscles together with parts of the tubules and an inner region or medulla that is devoid of renal corpuscles, and contains the remaining parts of the uriniferous tubules. Capillaries are abundant within both the cortex and medulla; lymphatics, however are NOT found in the medulla. Identify the cortex and medulla.
Click on link . The renal capsule surrounds the kidney. The lower center part of the image contains a medullary ray which is considered to be the center of a kidney lobule within the cortex. Zoom out once and click . Highlight the medullary rays which extend between the cortex and the medulla. The medullary rays are collections of straight portions of the thick descending proximal tubules, thick ascending distal tubules, and collecting ducts.. Also highlight the arcuate vein. The arcuate veins and arteries are located at the corticomedullary junction.
The functional unit of the kidney is the nephron. Note that the nephron and uriniferous tubule are often used as synonyms. Use schematic Fig. 1 (adapted with permission from Hospital Practice) as a reference. Each nephron consists of a renal corpuscle and a hollow tubule lined by simple epithelium. The epithelium varies in character within different segments of the nephron according to the function of the segment. Each nephron starts blindly in the cortex at the renal corpuscle and ends with a region known as the collecting duct. (Some consider the collecting duct not to be a part of the nephron, strictly speaking, since it has a different embryological origin.)
Go back to the low magnification view of slide w56a and follow [2,2]. Bowman's capsule and the glomerulus taken together comprise the renal corpuscle. Some authors use the terms renal corpuscle and glomerulus synonymously. Highlight these. Strictly anatomically speaking, the glomerulus is a tuft of capillaries which invaginates into the tubule. Bowman's capsule has two layers with an intervening space (Bowman's space) which opens directly into the proximal convoluted tubule. The inner (visceral) layer is composed of podocytes that coat the glomerular capillaries with their pedicles ("small feet"). Identify also the lumina of the proximal and distal tubules. The macula densa is part of the wall of the distal tubule where "it kisses its glomerulus good-bye" (C. C. C. O'Morchoe).
Zoom out once and then click link . Highlight the glomerulus, the proximal tubule lumen, and the urinary pole. Click link . In the renal corpuscle you should note the endothelial cell nuclei of the glomerular capillaries. Other nuclei visible belong to either podocytes or mesangial cells; it is not possible to distinguish between the two at the light microscopic level. The filtration barrier of the glomerulus (not visible in the light microscope) is composed of the fenestrated endothelium of glomerular capillaries, the basal lamina, the pedicles of the podocytes and the "slit diaphragm" between the podocytes. The mesangial cells "clean up" the basal lamina and sometimes regulate the lumen size of the glomerular capillaries.
The renal corpuscle has two poles, the vascular pole and the urinary pole, the latter being the site from which ultrafiltrate enters the proximal tubule. The proximal tubule is convoluted in the cortex, thus seen most often in cross-sections. The proximal tubule is the longest part of the nephron and reabsorpbs most of the substances (including glucose and albumin) from the urinary ultrafiltrate. The function of the brush border is to increase surface area for reabsorption. Highlight the brush border. The proximal tubule is the only part of the nephron having a brush border. This is helpful in distinguishing the proximal tubules from the other tubules. Cells of the proximal tubules have glycogen stores and basal cell membrane infoldings where mitochondria are located (not seen in LM). The proximal tubule descends to the medulla as the beginning of the Henle's loop (also called descending thick limb).
To see another example of the renal corpuscle with the above structures identified, click the thumbnail and follow links [2,3,1,1].
There are three types of nephrons in humans: cortical, short, and long (juxtamedullary). Cortical nephrons are restricted to the renal cortex. Short nephrons extend only to the outer medulla. Long nephrons reach the inner medulla, some even to the tip of the papilla. The long (juxtamedullary) nephrons are involved in the production of concentrated urine. This slide was made from the kidney of a stump-tail monkey and seems to lack a true inner medulla, which in humans contains only the thin limbs of the long nephrons. However, these monkeys concentrate urine as well as we do. In humans the cortical and short nephrons outnumber the long ones by a factor of 7 to 1. The short nephrons perform over 90% of the work of the kidney. For example, they resorb most of the glucose and other substances that need to be removed from the ultrafiltrate. They also receive over 90% of the kidney's blood supply.
Refresh your memory by reviewing Fig. 1. The loop of Henle is composed of the descending thick limb (straight proximal tubule), thin limb, and the ascending thick limb (distal tubule). In the short nephrons the thin portion is found only in the descending limb and the "loop" is composed of the thick ascending (distal) tubule epithelium. The long nephrons have a thin portion on descending and ascending limbs and the "loop" is lined by thin squamous epithelium. Go back to slide w56a. Click the thumbnail and follow [4,1]. Highlight the loops of Henle of short nephrons. Note that the epithelium at the loop is cuboidal distal tube epithelium. The thin descending limb is lined by squamous epithelial cells.
Highlight the distal tubules. They are lined by eosinophilic low cuboidal epithelium. Cells have plenty of mitochondria in the basal cell membrane folds. The distal tubule travels back to the cortex to touch its own renal corpuscle.
Click the thumbnail and follow [5,1]. Highlight the thin limbs of Henle. They are lined by squamous epithelial cells. One of these appears to be forming the beginning of the loop of a long nephron. The nuclei of the cells tend to bulge into the lumen. Highlight the vasa recta and note that their walls are even thinner than those of the thin limbs and their endothelial cell nuclei are fewer and further apart. Figure kidney1 shows additional views of the thin limbs of Henle of long nephrons and the vasa recta.
Go back to slide w56a. Click the thumbnail and follow [4,1]. Highlight the collecting duct. They play an important role inthe final concentration of urine. Collecting ducts join together to form larger ducts that eventually empty into a calyx. Click the thumbnail and return to [5,1] and highlight the collecting ducts. Click . Here you can see both light and dark cells lining the collecting duct. Click  for a higher power view of a collecting duct. Notice the abundance of lysosomes and peroxisomes in the cytoplasm of the cells lining the collecting duct. Notice also the thinness of the vas recta wall.
Click thumbnail and then [5,2]. Highlight the transitional epithelium. This is the lining of the minor calyx at the papilla.
Arcuate arteries run parallel to the corticomedullary junction. From the arcuates arise several interlobular arteries which run towards cortex in between two medullary rays. Interlobular arteries give off branches, the afferent arterioles, to the glomeruli. The afferent arterioles branch into an arterial capillary network (glomerulus). Each capillary network rejoins to form an efferent arteriole.
Each efferent arteriole leaves the renal corpuscle and then divides into one of two types of secondary capillary network. The efferent arterioles from the cortical and short nephrons are short and divide to form an eloborate peritubular capillary network in the cortex. Most of the efferent arterioles from the long (juxtamedullary) nephrons divide to form a bundle of parallel, non-branching arterioles, the arteriolar vasa recta. These travel into the medulla, make a hairpin loop, and return to the corticomedullary junction as venous vasa recta, which empty into the arcuate vein. The arteriolar endothelium is thicker than the venous and is continuous, whereas the venous endothelium is fenestrated.
Go back to slide w56a. Click thumbnail and [5,2] and highlight the vasa recta.
Click the thumbnail and follow [2,2]. Identify the macula densa. The macula densa is the tubular component of the juxtaglomerular complex. The vascular components are the afferent and efferent arterioles near their entrance to (or exit from) the glomerular tuft. The smooth muscle cells in the afferent arteriole wall are modified. They contain secretory granules representing renin or its precursors. The extraglomerular mesangial cells adjacent to the macula densa are also part of the juxtaglomerular complex. Identify them. Macula densa cells and the extraglomerular cells have intimate contacts with the arterioles. The juxtaglomerular complex is the place where the renin-angiotensin-aldosterone circuit is believed to be initiated. Zoom out once and follow [3,1,1] to see another view of the macula densa. Other examples of the juxtaglomerular complex are shown in Figures kidney2 and kidney3. Identify macula densa, extraglomerular mesangial cells, and arteriole (one cannot tell if it is afferent of efferent). In kidney3 note also a part of the wall of the interlobular artery.
Click [1,3,1]. Identify the transitional epithelium, which occurs only in the urinary tract. The cells are stacked five or six cells thick and the surface is in folds. When the lumen fills the folds flatten and the epithelium stretches, becoming only one to two cells thick.
Click the thumbnail and link [1.2]. The smooth muscle is arranged somewhat indistinctly as an inner longitudinal layer and an outer circular layer.
Practice and self assessment excersise: Find the transitional epithelium in this unlabeled virtual slide: Urethra, male, dog.
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