Urinary System

OBJECTIVES: At the end of this laboratory, you should be able to:

Identify the kidney, ureter, and urinary bladder microscopically.
Distinguish between the cortex and medulla of the kidney.
Identify the segments of the nephron; i.e., the proximal tubule, distal tubule, and Bowman's capsule with the glomerulus and the loop of Henle.
Identify medullary rays, distinguishing them from other regions of the kidney cortex
Identify the arteries, arterioles, capillaries, and veins, relating them to the nephron and collecting ducts.
Identify the macula densa and understand its relationship to the glomerulus.
Identify the urinary and vascular poles of Bowman's capsule.
Correlate the light microscopy of each part of the nephron with its ultrastructure.

Kidney

Kidney
Observe the cortex, medulla, and delicate connective tissue capsule. Note the relatively large arteries and veins between the cortex and medulla. These are the arcuate arteries and arcuate veins. Recall that arcuate arteries give rise to interlobular arteries which run through the cortex and that interlobular veins drain into arcuate veins. From the interlobular arteries, the afferent arterioles branch off to supply the kidney glomeruli. Identify the large interlobar arteries that give rise to the arcuate arteries.
Renal corpuscles (glomerulus + Bowman's capsule) as well as the proximal and distal convoluted tubules are found in the cortex. One can distinguish proximal convoluted tubules from distal convoluted tubules by these characteristics:
1) Staining - proximal tubules are more eosinophilic than the distal.
2) Nuclei - proximal tubule cells have more heterochromatic nuclei than those of distal tubule cells; and there are more nuclei per unit area in the distal tubule than there are in the proximal tubule.
3) Cytoplasm - the distal tubule cells appear with less cytoplasm.
4) Microvillus "brush" border - in the proximal convoluted tubules the “brush” border has sloughed and can be seen in the lumen; thus, the lumen appears partially filled.
Observe the medullary rays and identify collecting tubules within the rays. Identify the vascular pole and urinary pole of the renal corpuscles. Also observe the parietal layer of Bowman's capsule, the urinary space, and podocytes. Find the macula densa of the distal convoluted tubule.

Kidney
In this specimen, the tubules, glomeruli, and capsule are better preserved. Identify proximal convoluted tubules and distal convoluted tubules. The proximal tubule nuclei are smaller and stain more intensely than those of the distal tubules and the proximal tubules cells are larger and have a “brush border” comprised of microvilli on their apical surface. Identify the parietal layer of Bowman’s capsule and the podocytes which line the urinary space. Observe the vascular pole and the urinary pole of the renal corpuscle. Identify the macula densa, which is located in a portion of the distal convoluted tubule that is in close proximity to the renal corpuscle which contains the glomerulus. Identify interlobular arteries in the cortex that supply the afferent arterioles.

Observe the juxtaglomerular cells (”JG cells”) in the wall of the afferent arterioles. JG cells are modified smooth muscle cells that secrete renin. These cells can be readily identified in this preparation by the intensely eosinophilic staining of the renin granules in their cytoplasm.

Identify the arcuate arteries which give rise to the interlobular arteries near the border of the cortex and medulla. Observe collecting ducts in the medullary rays of the cortex.
Study the medullary region and note the vasa recta, thin limbs and thick limbs of the loop of Henle, and collecting ducts. In some preparations such as this one, the vasa recta can be readily distinguished from the thin limb of the loop of Henle by the appearance of red blood cells in the lumen. Collecting ducts in the medulla are distinguished by the clarity with which the borders of adjacent lining cells appear.

Kidney, mammal (medial sagittal section)
In this specimen, observe the major structures of the kidney, such as the major calyces and papilla. Observe the collecting ducts (of Bellini) in the papilla that receive the urine from the collecting tubules of the medulla. Observe that the renal pelvis gives rise to the ureter. Note that the lumen of the ureter is lined with transitional epithelium.

Ureter

Ureter
Examine the layers of smooth muscle. An inner longitudinal layer and outer circular layer are distinguishable in this specimen. Note the lamina propria. The obvious spaces in this layer are artifact caused by excessive shrinkage and extraction of ground substance. Note that the lumen of the ureter is lined with transitional epithelium. The “dome” cells of the epithelial surface have eosinophilic dense plaques on the apical membrane.
Ureter
This is a special preparation in which a more stretched ureter is placed next to a more contracted one. Note the morphology of the transitional epithelium in the distended and partially relaxed states and compare both of these with the nearly fully contracted ureter in the previous specimen.

Urinary Bladder