Slide #107 shows proximal and distal sections of a ureter. Identify the layers, noting especially the transitional epithelium. How can you tell which one was nearest the kidney and which one was nearest the bladder? Also, do you see a serosa?
Slide #14 is a section of bladder with a piece of ureter included in the lumen strangely enough. Review all layers of both structures. The bladder epithelium appears better preserved than that of the ureter. Note the dense irregular connective tissue of the lamina propria, and in the bladder, its vascularity. How many muscle layers are there in the muscularis? Is there a serosa present?
Arcuate veins will appear as large, empty circular profiles and serve to delineate cortex from medulla, which can't be distinguished grossly on the basis of texture or color. The medulla is subdivided into several zones and stripes. The outer zone of the medulla is clearly distinguishable from the inner zone, which is light and has a fine texture. The inner zone is the portion referred to grossly as the papilla. The outer zone is further subdivided into stripes. The outer stripe is very eosinophilic due to the presence of large, eosinophilic proximal tubules. The inner stripe paler due to absence of proximal tubules. Thus, while the outer medulla is two-toned with its two stripes, the inner medulla appears as a more homogeneous area.
Begin your study of the nephron in the inner medulla (papilla), and work your way out to the cortex. The medulla has the simplest structure and fewer nephron types, whereas the cortex is more complex.
Even to the experienced, not all tubule types are identifiable in paraffin sections! If some tubules are indeterminant, as many will be, don't concern yourself, but concentrate on being able to identify good examples of each structure underlined.
In the rat kidney, the collecting ducts are lined by a flatter epithelium, and the best way to identify them is by their wide lumens. You will not be tested on the appearance of rat collecting duct since it is not representative of human histology.
Thin limbs and capillaries again are difficult to distinguish from each other, but tend to aggregate together in bundles, separate from the thick ascending limbs. As usual, they are lined by a squamous epithelium.
Look for collecting ducts as they course through this stripe.
Identify the components of the renal corpuscle: Look for afferent/efferent arterioles - you can't tell them apart, but you can see them entering Bowman's capsule at the vascular pole. Find the glomerulus, parietal and visceral layers of Bowman's capsule separated by Bowman's space. At the urinary pole the parietal epithelium abruptly changes to the tall epithelium of the proximal tubule. Identify glomerular capillaries, and nuclei of podocytes (remember these are the cells of the visceral epithelium) vs. endothelial cells. Identify intraglomerular and extraglomerular mesangial cells. The latter can only be appreciated when both entering and exiting arterioles are in the plane of section at the vascular pole. These extraglomerular mesangial cells will be found between them.
Look for proximal tubules in the rat kidney first, focusing up and down to resolve the brush border. These tubules will appear collapsed and fuzzy in the human kidney, and certainly a brush border is not resolvable. Convoluted and straight segments appear identical, but the straight, or pars recta, sections will be seen coursing in long rays parallel to thick ascending limbs and collecting ducts. Cells of proximal tubules are low columnar to cuboidal; their nuclei are located in a central position in each cell.
Whereas the thick ascending limb displays a tall cuboidal epithelium in the outer medulla, as it approaches the glomerulus it becomes low cuboidal to high squamous. Search the corpuscles for fortuitous sections showing an approaching thick ascending limb. At a variable distance from the glomerulus, the epithelium makes a quick transition to a cuboidal epithelium again. This is the beginning of the distal convoluted tubule. Distal convoluted tubules lack a brush border and have dense, apically-located nuclei.
Be sure to find a macula densa. This will be seen as a cluster of cells in the thick ascending limb where it passes between the afferent and efferent arterioles, forming the juxtaglomerular apparatus. In the human, these cells are cuboidal to columnar and tightly packed. In the rat the cells of the macula densa are flatter. In either case, the close packing of the cells causes their nuclei to form a characteristic dense area, giving rise to the term macula densa, or dense spot.
To find cortical collecting ducts, look for longitudinally oriented tubules running in rays perpendicular to the surface of the kidney. The epithelium of these tubules in the cortex appears fairly flat, with a wide lumen, thus do not expect them to look like they do in the inner medulla. The hallmark of the cortical collecting duct is the presence of two cell types. Most of the lining cells will be flat--these are the principal cells. Interspersed among these are plump intercalated cells.
The space between the tubules is the interstitium. It is quite sparse and occupied primarily by peritubular capillaries.