Slide #DMS 152 [Rabbit; Kidney, pelvis and ureter, l.s., H&E]. Orient yourself to this slide by identifying the features with which you are already familiar. This slide shows a partial longitudinal section of a unilobar kidney. It is a relatively thick section but shows some details of kidney structure very well. Hold the slide up to light and and view with your unaided eye or with an inverted ocular. You can readily identify cortex, and the various zones of the medulla: outer medulla (further divided into outer and inner stripes), inner medulla, papilla, and then calyx and ureter. Switch to the high power and focus on the papilla, calyx, and renal sinus regions. The transitional epithelium (urothelium) covering the papilla and lining the calyx is well preserved in this specimen. Look for segments of the ureter that drain the calyces. What type of lining epithelium do they present?
Now return to your study of the uriniferous tubule. First, identify the renal corpuslces, which, in this preparation, are not so elegantly dislpayed as in Slide #DMS151. Now turn your attention to the tubular system of the cortex. Although better preservation of tubular architecture will be seen in subsequent slides, some of the fundamentals may be appreciated here. Identify the proximal convoluted tubules, the profiles of which dominate the cortical labyrinth. Study the epithelium for the following characteristics: size and shape of cells, density of cytoplasmic granulation and presence of a brush border. (The brush border is very sensitive to fixation conditions and was not well-preserved in this section). How many nuclei are usually found in a cross-section of the tubule? In a medullary ray identify the straight part of the proximal tubule (aka, thick descending limb of loop of Henle) and note that the cytology is the same as that of the convoluted portion.
In the outer zone of the medulla identify the thin limbs of the loop of Henle and contrast the size of the tubule and thickness of its epithelium with those of neighboring capillaries. (The capillaries usually contain some RBCs). The thin portion of Henle's loop is especially clear in the inner zone of the medulla.
In the outer medullary zone and in a medullary ray, try to find the thick part of an ascending thick limb of Henle's loop (aka: straight part of distal tubule). Compare its histology with that of the proximal segment of the tubule. These differences can be discerned quickly in the medullary ray because the distal and proximal segments are the only parts of the uriniferous tubule present whose epithelial cells have a granular cytoplasm.
Review how the arrangement of the different segments of the loop of Henle in relation to vasa recta establishes a "countercurrent exchange system" for adjusting the osmotic concentration of the blood in the medulla.
Study the distal convoluted tubule in the cortical labyrinth. Except for the convoluted form, its structure is identical with that of the thick ascending portion of Henle's loop in the medullary ray. Again, the preservation of cellular architecture is not optimal in this section, and some of these features will be more apparent in subsequent slides.
Collecting System. Within medullary rays find cortical collecting tubules. In comparison with the proximal and distal convoluted tubule, note the caliber of the lumen, uniformity in height of the epithelium and size of its cells, cytoplasmic homogeneity, and distinctness of the cell membranes. The major changes seen as they continue into the medullary collecting tubule s and then the papillary ducts are a gradual enlargement of the lumen and an increase in the height of the lining epithelium. Locate an opening of papillary duct into a minor calyx at the area cribrosa.
Observe the character of the interstitial connective tissue in relation to tubules and blood vessels. In which part of the kidney is connective tissue most abundant?
Human Kidney: 1. Cortex; 2. Renal columns; 3. Medulla; 4. Papilla of medulla; 5. Calyx; 6. Renal pelvis.
This is a very low power view of a section through the unilobar kidney of a rabbit. The cortex and medulla are well-defined, as are the subdomains of the medulla which include the inner medulla and the outer medulla, the outer medulla being further subdivided into an inner stripe and an outer stripe. Note again the prominent arcuate vessels (artery & vein) found at the border between cortex and medulla.
A low power view of the cortex reveals many of the same features as seen previously including medullary rays and the elements of the cortical labyrinth found between the rays. Numerous renal corpuscles are seen here, though not as well as in the previous slide since the urinary (Bowman's) spaces are collapsed in this preparation.
At somewhat higher power, one may better appreciate the longitudinal orientation of the tubules making up the medullary ray, the numerous cross sections of proximal and distal tubules that fill the cortical labyrinth, along with a couple of renal corpuscles. Note also an interlobular artery.
In this high power view of the cortical labyrinth, one may distinguish, albeit not optimally, proximal and distal convoluted tubules. Sections through the proximal convoluted tubules, characterized by large, eosinophilic cells, outnumber the profiles of distal convoluted tubules (smaller, paler-staining cells) roughly 7:1. Again notice that the distal tubules seem to contain more nuclei than a comparable section through a proximal tubule.
In this high power image, one may compare numerous proximal tubules with a small collecting duct. The collecting tubules and ducts are most easily distinguished by the prominent lateral borders seen between constituent cells. Neither the proximal nor distal tubule cells typically show such prominent lateral margins.
This is a low power view of a portion of the renal pyramid and its papilla. Even at this low magnification, the large collecting ducts (of Bellini) draining towards the papilla are evident. Also appreciate the increasing amount of interstitial tissue between tubules as one progresses deeper into the medulla.
A higher power image from the outer medulla reveals cross sections through three different tubule populations: distal tubules (here also called thick ascending limbs of the loop of Henle), collecting ducts, and thin limbs of the loop of Henle (lined by simple squamous epithelium). Some peritubular capillaries may also be distinguished (blood in their lumina). Again note the more well-defined lateral borders distinguishing the collecting tubules from the distal tubules. This section also demonstrates well the tendency for nuclei in the distal tubules to be apically disposed.
This is a medium power view of the papillary region of the renal pyramid. Note the large collecting ducts (papillary ducts of Bellini) which have now assumed a simple-columnar morphology. These ducts will open onto the surface of the papilla, though this continuity is not seen in this image. Capillaries, thin limbs of the loop of Henle, and abundant interstitial tissue would be seen at higher mag in the space between the collecting ducts.
This is a higher power view of the inner medullary tissue showing the simple columnar epithelium of the collecting ducts found here. Note again the more prominent cell boundaries that help to distinguish this tubule from the other tubules of the kidney. Peritubular capillaries and a thin limb of the loop of Henle can also be identified in this image. While these latter two structures may be confused if the capillaries are not filled with blood, note also the plumper nuclei in the thin limbs compared with the very fusiform endothelial cell nuclei of the capillary.
In this image of the papilla of the renal pyramid and the calyx draining it, note that transitional epithelium forms both the surface epithelium of the renal papilla and the lining of the calyx.
A higher power view of the wall of the calyx demonstrates the transitional epithelium (urothelium) forming its mucosal surface and the smooth muscle contributing to the wall of the calyx. This smooth muscle in the wall of the calyx may help to 'milk' urine out of the renal papilla and into subsequent ductwork of the urinary system.