URINARY SYSTEM
Objectives
You should be able to identify the following in the class slide
sets:
| Kidney | proximal convoluted tubule | |
| capsule | proximal straight tubule | |
| cortex | thin limb of loop of Henle | |
| labyrinth | distal straight tubule | |
| medullary ray | distal convoluted tubule | |
| medulla | collecting duct | |
| renal papilla | ||
| calyx | Ureter | |
| lobe | transitional epithelium | |
| lobule | muscle layers | |
| arcuate artery | ||
| interlobular artery | Bladder | |
| afferent/efferent arteriole | stretched | |
| peritubular capillary | relaxed | |
| vasa recta | transitional epithelium | |
| nephron | umbrella cells | |
| renal corpuscle | muscle layers | |
| Bowman's capsule | ||
| parietal layer | Urethra (female) | |
| visceral layer | various epithelia | |
| glomerulus | smooth muscle | |
| juxtaglomerular apparatus | ||
| macula densa | Urethra male | |
| mucosa | ||
| submucosa (erectile tissue) |
 |
Blue Histology |
Slides
 | D-42 | Kidney, monkey (FeH) | ||
| D-43 | Kidney, monkey (H&E) | ||
| D-44 | Kidney (H&E) | ||
| D-45 | Kidney (PASH) | ||
| D-47 | Ureter (H&E) | ||
| D-48 | Bladder, stretched (H&E) | ||
| D-49 | Urinary, bladder (H&E) | ||
| D-50 | Female urethra (H&E) | ||
| D-71 | Penis (H&E) |
Prologue
Your slide collection contains four slides of kidney. Each is included for a specific purpose. D-43 was prepared from well-preserved animal kidneys and stained with H & E. It is the best slide for studying the various blood vessels and ducts of the kidney. D-44 is a corresponding section of human kidney. As you will note, its state of preservation is not as good as the monkey tissue. However, it allows you to examine a typical slide of human material. Slides D-42 and D-45 represent special stains. The iron hematoxylin stain of D-42 brings out subcellular details. The various cell types of the nephron are worth viewing with this stain. D-45 is stained to reveal basement membrane (which is pronounced and extremely important to this epithelial organ). You have already examined this slide during an earlier laboratory session.
Small kidneys consist of only a single lobe. A section across the whole organ
is provided on slides D-42 and D-43. Only a part of the human kidney can be shown
on slide D-44. Do not feel gypped if it does not show a whole lobe or interlobular
structures. Kidneys are quite large organs
Slide descriptions
D-43, Kidney (H&E)
By eye locate the major gross divisions of the kidney: capsule, cortex, medulla,
renal pyramid, calyx and hilus. Using your low-power objective distinguish cortex
from medulla and medullary rays from areas of labyrinth in the cortex. Your textbook
presents a clear description of the position of the nephron in the kidney. Each
nephron consists of a renal corpuscle, a proximal tubule with a convoluted and
straight portion, a thin limb of the loop of Henle and the convoluted and straight
portion of a distal tubule. Figure out, if it is not already clear to you, why
the distal straight tubule is sometimes called the thick ascending limb of the
loop of Henle. List the structures you expect to find in the:
| 1. labyrinth area (high mag) 2. medullary ray (high mag) |
_____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ |
With the aid of the text, identify the various segments of the nephron and the collecting ducts in each area. The only difficult distinction should be between the thick ascending limb of the loop of Henle and the collecting duct in the medullary ray (illustration).
Renal corpuscles show up well. Spend some time comparing their appearance with the diagrams in your textbook. In some cases you should be able to see the simple squamous epithelium of Bowman's capsule, lining the urinary space. It is not feasible to distinguish podocytes from endothelial cells in the glomerulus. Nor can one satisfactorily identify the mesangeal cells in a thick section, such as this.
When you find a properly oriented section, you will be able to recognize the macula densa (second example)It appears as a clustering of nuclei on one side of a distal tubule which lies against the vascular pole of the renal corpuscle. You probably will not be able to recognize the juxtaglomerular cells of the afferent arteriole. But you should be able to regurgitate the function of the JG apparatus?
The blood supply in the kidney is highly important. It is unusual in having an arterial portal system. (What are the five components of a portal system?) If you search in the appropriate regions of your slide, you can see sections of most of the named divisions of the arterial supply. Identify interlobar arteries located in the pelvis region. Since this kidney has only one lobe these arteries are the equivalent to primary branches of the renal artery (labeled above). Arcuate arteries and veins run between the cortex and medulla. You can see them as tiny white holes if you hold the slide up to the light. Hold each of the other slides up similarly and note them there as well. Slide D-44 shows them especially well as a line of holes along the medullary-cortex border. Examine them under low power and distinguish the adjacent artery and vein (illustration). Interlobular arteries branch off from the arcuate arteries and course upwards, in the plane of section towards the capsule. They run along the boundaries between lobules, hence their name. Your section will cut through an occasional one with the typical appearance of a small artery (example).
This is an appropriate time for you to consider the lobular structure of the kidney. A lobule is the cone shaped unit of tissue which is involved in secreting urine from one papillary collecting duct. A medullary ray lies in its center and the labyrinth area containing the rest of those nephrons form the outer part. The interlobular arteries lie between lobules and give off tiny afferent arterioles which run into the lobule to supply the renal corpuscles.
The afferent arteriole enters a renal corpuscle, breaks up into a tuft of capillaries called a glomerulus, and then collects back into an efferent arteriole instead of a vein. With sharp eyes and a rabbit's foot for luck you may see one of these arterioles, but it will be impossible to tell whether it is afferent or efferent. The efferent arterioles run a short distance and then break up into a second bed of peritubular capillaries, which nourish the proximal and distal tubules and pick up fluid. Even when these capillaries are collapsed you can easily see their abundant flattened nuclei between the convoluted tubules (illustration).
The efferent arterioles from those nephrons situated near the boundary between the cortex and medulla take a different course. They break up into long, thin capillary vessels which descend straight down into the medulla, and then loop back to the boundary. These capillaries are called the vasa recta. They provide nourishment to, and resorb water from, the medulla. The best place to observe the vasa recta is in a region of medulla which is cut perpendicular to the tubules. But wait until you locate these vessels in slide D-45 stained with PASH and then you will have little trouble finding them on this slide.
Since this slide contains a complete section of a small kidney, the pelvis region is easily seen and understood. The ureter enters and its expanded end pushes up against the medullary papilla to form a calyx with a visceral layer (connected to the papilla) and a free parietal layer. Both layers are lined by transitional epithelium, recognizable by the darker staining top layer of cells some of which have two nuclei. This epithelium lines the entire urinary passageway down to, and including part of, the urethra. For larger kidneys with multiple lobes the ureter will form a separate calyx over each renal papilla.
Take a look at the very large terminal portions of the collecting ducts. On my slide some of them are composed of pseudostratified epithelium with patches of stratified cuboidal epithelium. Can you make these distinctions on your slide? If you are lucky you will see one or two ducts emptying through the epithelium lining the papilla into the calyx.
So, finally here is a little quiz. Where was this tissue taken from, be specific and identify each of the tubules shown.
D-42 Kidney monkey (FeH)
Slide D-42 in an exceptionally fine slide. It has a section of a well preserved unilobular kidney cut thinly and stained with iron hematoxylin. Although the stain is just "black and white" it emphasizes cytological detail. The lobular organization is especially visible as you scan around the cortex at low power (illustration). Check your identifications for the various parts of the nephron and collecting ducts. Find a macula densa. If you are compulsive enough to need to see afferent arterioles this is the slide to look at. The arterioles are small but you can recognize them by their elongated endothelial nuclei. It is very difficult to tell afferent from efferent arterioles but in one place I was able to make out the JG cell in their walls (illustration).
The pelvis, calyx with transitional epithelium, an "interlobar artery" (= renal artery for this unilobular kidney) and arcuate vessels show up well. Also there is a section though the ureter just outside of the pelvis. Do not miss examining it. It shows excellently preserved transitional epithelium in the stretched configuration right next to relaxed areas.
D-45 Kidney human (PASH)
The PASH stain emphasizes the basement membranes which are particularly prominent in the kidney. The cells have a very washed out appearance (illustration) but the entire organization of the kidney is graphically outlined by basement membranes (illustration).
Look for four things:
1. The basement membrane of the glomerulus (illustration). You are aware of the importance of the basement membrane to the renal function. With this membrane so well stained it is just possible to recognize some podocytes lying on the outside of it. In most H&E preparations it is impossible to distinguish them from the endothelial cells lying under the basement membrane. Note as well, that the entire nephron is surrounded by a very well developed basement membrane.
2. The brush borders of proximal tubules. These structures are rich in glycocalyx which stains with PAS. You already looked at them on this slide in a very early laboratory exercise. Now review them as excellent presentations of a tall brush border.
3. Vasa recta. You can see nicely the disposition of the vascular supply to the medulla of the kidney. At low power find an area of the medulla in which the tubules are cut in cross-section. Note that some small, scattered areas here have more basement membrane and appear darker or more cluttered than the surrounding areas (illustration). These areas contain clusters of vasa recta. Switch to higher power and confirm this by noting the red blood cells in the vessels (illustration).
4. (For the diligent only:) Arterioles entering/leaving renal corpuscles. Arterioles happen to be well distended on this slide and relatively easy to see. You can see the elongated smooth muscle cells in their walls in appropriately oriented sections (illustration).
Now that you know how vasa recta are disposed in the medulla, return to slide D-43 to view them stained with H & E (illustration). Obviously, if you are interested in looking for sections through the thin limb of the loop of Henle you should avoid these areas where the vessels of the vasa recta might confuse you. Compare these large capillaries at high power with nearby thin limbs of Henle. They are of about the same caliber, but the thin limbs have somewhat thicker walls and no blood cells inside of them.
D-44 Kidney Human (H&E)
Slide D-44 shows you a human kidney stained with H&E. Our kidneys are multi-lobed and so large that not even one lobe will fit in its entirety on a slide. Otherwise, this section does not differ significantly from those of your monkey tissues. Review the parts of the nephron, assuring yourself that you can distinguish the various tubule types.
One nice thing about this slide is that the tissue has retained quite a bit of blood. Enough red blood cells are left in its vasculature for you to recognize the vasa recta, peritubular capillaries and some afferent/efferent arterioles.
D-47 Ureter (H&E)
At low power, observe the three layers of the ureter: mucosa, muscularis and adventitia (illustration). This tripartite organization is maintained throughout the urinary passage system. The mucosa is thrown into characteristic longitudinal folds. It is made of transitional epithelium underlain by a layer of fairly cellular connective tissue. Quite a few lymphocytes are found in this lamina propria. A lymph nodule may be seen in some slides. Does yours have one? The muscularis layer consists of two somewhat diffuse sheets of smooth muscle: the inner is longitudinal, and the outer is circular. Towards the bladder end it picks up a third oblique layer. This muscle produces peristaltic waves that push the urine to the bladder even when you are lying down. The adventitia is made of connective tissue and contains the nerves and blood vessels that supply the organ. One thing to note is that the ureter runs through loose connective tissue from the kidney to the bladder. You can see some remains of it on the surface of the organ.
D-49 Urinary bladder (H&E)
Lower power: the layers of the bladder are basically the same as in the ureter (illustration). However, the muscle layer is much thicker and includes a third sheet of smooth muscle with oblique orientation. Of course for a spherical organ like the bladder "longitudinal", "circular", and "oblique" are somewhat formalistic terms. In any case the sheets of smooth muscle are indistinct, so do not waste time trying to distinguish them. This is a good slide to use to study transitional epithelium. Look particularly at the umbrella cells and their acidophilic apical borders. By examining the epithelium, determine the state of distention of the bladder.
D-48 Bladder stretched (H&E)
You have just looked at transitional epithelium of the bladder in the contracted state. In preparing D-48 care was taken to stretch the tissue while it was being fixed (by about as much as two Australian beers and a bag of chips would.) Look in particular at the mucosa. In some places the epithelium is not as well preserved as one might wish but it is obviously stretched. There the umbrella cells have acquired a squamous shape.
Now here is a question for you to answer by looking at your slides of the bladder. When the epithelial cells are stretched into a squamous configuration do the nuclei also flatten out?
The muscle layer is still a bit of a mess on this slide instead of three nice layers as some of your atlases would lead you to believe.
Most of the bladder is covered by an adventitia; however, part of the organ has a free surface in the body cavity. Here a layer of mesothelial cells covers the connective tissue. This layer is then called a serosa. Its mesothelium is part of the peritoneum that lines the entire body cavity and the organs which protrude into it. Does adventitia or serosa cover the pieces of the bladder on your two slides?
D-50 Female urethra (H&E)
Under low power locate the urethra. It repeats the same three layers as the ureters, but is thicker (illustration). The two muscle layers are somewhat indistinct. The epithelium will vary according to the region from which the section is taken. Near the bladder it is transitional, at the external opening it is stratified squamous, and between those two places the epithelium is pseudostratified or stratified columnar. What is your call on your slide (illustration)?
Notice the small outpocketings of mucus-secreting cells which form clusters in the epithelium and extend into the lamina propria as simple glands. These are called the lacunae of Morgagni. Those in the epithelial layer are examples of intra-epithelial glands.
How will you distinguish the urethra from the ureter on the slide identification part of the final? (Now is the time to think about this.)
D-71 Penis (H&E)
We will study the penis in more detail in the laboratory on the male reproductive system. Its reproductive function is more interesting than its urinary function (despite what your mother may have told you). Only the urethra concerns us at the moment. Locate this structure in the spongy portion of erectile tissue (illustration). It is lined with stratified or pseudostratified columnar epithelium with patches of stratified squamous epithelium (illustration). As the urethra broadens to its terminal enlargement and opens at the apex of the penis, the epithelium becomes predominantly stratified squamous in form. Notice the epithelial outpocketings for mucous glands, called the Glands of Littre. These are actually easier to distinguish than the lacunae of Morgagni in the female urethra. The names of these urethral glands are unimportant. In fact, the glands probably are not very important, just providing a bit of protection to the non-transitional epithelium that is exposed to urine.