- Understand the organization of the renal corpuscle and the cells present within it.
- Describe the filtration barrier between blood and urine in the renal corpuscle.
- Name the divisions of the nephron, and specify their locations (pars convoluta or medullary ray of cortex, or medulla).
- Relate the histological specializations found in specific divisions of the nephron to the functions of that division.
- Describe the blood supply of the kidney.
- Describe what structures are involved in regulation of blood pressure.
In this laboratory period you will study the kidney, ureter and bladder. Your main goal in the kidney will be to understand how blood vessels and nephrons are organized in the kidney cortex and medulla, and how this arrangement is related to the production of urine. The nephron consists of a renal corpuscle (the glomerulus and Bowman's capsule), a proximal tubule (with convoluted and straight portions), a thin segment, and a distal tubule (with straight and convoluted portions). The nephron empties into a collecting duct. You will not be able to see a nephron in its entirety in a single section, but will study the various portions separately in the slides below. Unfortunately, the kidney is difficult to fix optimally, and so you will look at several kidney slides, each of which may show one aspect or another to better advantage. The quality of fixation may even vary somewhat in different parts of the same section, so look around for a moment before you settle down to study structures. Consult Figs 16.2-16.4 (Wheater's pgs 303-304) for orientation diagrams.
Part of a human kidney in cross-section is shown in slide 204. One side of the section is relatively smooth and convex; this is the outer surface of the kidney. Underlying that surface (capsule) is a layer of cortex [example] about 5 mm thick. Most of the remainder of the section is the medulla [example], forming renal pyramids (roughly triangular in appearance). The apex (tip) of the pyramid forms the papilla [example].
The short red strips, which you may see in the medulla in slide 204 are "vasa recta" [example] (see section "C" below for more discussion of the kidney vasculature). The monkey kidney (slide 210) is a "unipyramidal" type--it has only one pyramid; the human has many. The monkey kidney was perfused and most of the RBCs have been washed out, but the histology is excellent and the diameter of tubules is close to that in real life.
Examine the cortex of slide 204 (W pg 306, 16.5). You will recognize medullary rays (or pars radiata) [example], which are clusters of parallel tubules (sectioned longitudinally) that appear to be coming out from the medulla. The region of cortex between the rays, called the cortical labyrinth (or pars convoluta) [example], contains renal corpuscles and the convoluted portions of the tubules.
1. Tubules (W pgs 315-17)
Identify the three general types of tubules that occur in the cortical labyrinth and medullary rays of the cortex:
- proximal tubules (further sub-divided into convoluted and straight portions),
- distal tubules (also divided into convoluted and straight portions), and
- collecting tubules (or ducts).
The preservation of tissue varies between the two slides. A certain degree of distortion and tissue breakdown exists and it will be necessary to study both slides for the best histology of the tubules. Most of the tubules you see in the cortical labyrinth in slide 204 [example] and slide 210 [example] are proximal convoluted tubules, which are large, prominent and generally stain a deeper pink than the other tubules. As an artifact of histological preparation, in some sets there may be small, white splits in the walls of these tubules, which should be ignored. In the cortex in slide 204, the straight portion of the proximal tubule [example] is in the medullary rays, and has a similar histological appearance to proximal convoluted tubules. It is difficult to have the brush border (composed of microvilli) well preserved in microscopic preparations. The brush border on the luminal surface of the proximal tubule epithelium in slide 204 is less well preserved than in slide 210 and tends to slough off and partly fill the lumen as pink material. In slide 210-PAS, stained with periodic acid-Schiff reagent, there is good preservation and staining of the brush border. Be sure you actually see the brush border. In addition, the basement membranes associated with the epithelial linings of blood vessels, Bowman capsules, and tubules are distinct [example].
Here and there among the proximal tubules in the cortical labyrinth you will also see distal convoluted tubules in slide 204 [example]and slide 210 [example]. You should note that distal tubules are paler in appearance, usually have a smaller diameter, and a low cuboidal epithelium. In the cortex, the straight portion of the distal tubule [example] is similar in appearance and occurs in the medullary rays.
The third type of tubule in the cortex is the collecting duct (or tubule), which is also best seen in the medullary rays in slide 204 [example]and slide 210 [example]. Look for tubules in which the epithelium is simple cuboidal or low columnar, the cell outlines usually appear particularly distinct, and the nuclei are prominent and closer together than in proximal or distal tubules. Be sure you can identify each of the three types of straight tubule found in a medullary ray [example] (proximal straight, distal straight, and collecting tubules). Collecting tubules may also be seen occasionally in the cortical labyrinth. Numerous capillaries occur between the tubules in the cortex. In slide 204, note the outlines of red blood cells (example) in these small vessels. The kidney in slide #210 was perfusion-fixed and, therefore, the capillaries are devoid of red blood cells.
2. Renal Corpuscle (W pg 307, 16.7; pg 308, 16.9; pg 315, 16.17; pg 318, 16.19)
Examine the renal corpuscles found in the cortex, noting the numerous capillary loops of the glomerulus [example]. Most of the flat nuclei in the glomerulus belong to endothelial cells and to podocytes (simple squamous epithelium constituting the visceral layer of Bowman's capsule). Some nuclei in the central regions of the glomerulus may belong to mesangial cells. What are the three layers involved in glomerular filtration and how do they work? (UR1) (Note that we do not expect you to be able to distinguish among these 3 cell types by light microscopy). The parietal layer of Bowman's capsule is also a simple squamous epithelium which transitions to cuboidal epithelium of the proximal convoluted tubule at the urinary pole (click here for an example). Look around under low power to find glomeruli sectioned through the vascular pole. Near the vascular pole will be the distal tubule of the same nephron. Some sections in slide 204 [example]and slide 210 [example] will show a portion of this distal tubule with unusually closely packed nuclei. This region of the distal tubule is the macula densa (example) which signals to the juxtaglomerular cells (modified smooth muscle cells that make renin) in the wall of the afferent arteriole. The macula densa, juxtaglomerular cells, and cells of the extraglomerular mesangium constitute the so-called "juxtaglomerular apparatus". You cannot distinguish juxtaglomerular cells in these preparations (but you could detect them by immunological techniques, e.g. immunostaining for renin).
B. Medulla (W pg 306, 16.5; pgs 320, 16.20; 321, 16.21; 326, 16.24)
Move to the medulla [example], where straight proximal and distal tubules as well as collecting ducts are found. Blood vessels (note outlines of red blood cells in slide 204) are also seen. In the medulla is the loop of Henle, usually composed of:
- An initial thick portion that represents the continuation of the straight proximal tubule from the medullary ray,
- A thin descending portion that turns back toward the cortex as a thin ascending portion that is continuous with
- A thick ascending portion, which is a segment of the straight distal tubule.
The thick portions have an histology characteristic of either proximal or distal tubule. The thin portion is lined by a simple squamous epithelium and cannot reliably be distinguished from capillaries (unless blood cells are present in the capillaries as in slide 204 [example]). The deepest portions of the medulla have only thin segments and collecting ducts. The epithelium of the collecting ducts becomes higher as these ducts pass toward the papilla (where they are called "papillary ducts" or ducts of Bellini [example]). As an artifact in some slides, the collecting duct epithelium may be pulled away from its basement membrane in some areas of the papilla, leaving a white space between the epithelium and its underlying connective tissue. Urine is released at the papilla through 10-25 openings (area cribrosa) into one of the minor calices which you will note are lined with transitional epithelium [example] (somewhat damaged in slide 204 [example]) as is the rest of the urinary tract. It is worth noting that, from this point onward, the osmolarity of the urine can no longer be modified very much since transitional epithelium is essentially impermeable to salts and water.
C. Blood Supply (W pgs 304; 306; 308-9)
Now that you have seen the arrangement of various nephron components in the kidney, go back and follow the blood supply. Slide 204 is helpful to study the blood supply even though the tubular epithelium in this slide is in bad shape! You will remember from gross anatomy that the renal artery enters the hilus of the kidney, and divides successively into lobar, interlobar (these are difficult to identify with certainty in histological sections, but they are the large arteries among the pyramids -UPSTREAM of the arcuate arteries) and finally into arcuate arteries, which are accompanied by the corresponding veins.
Observe arcuate arteries and veins [example], sizeable vessels passing along the boundary between the cortex and medulla. From the arcuate arteries, relatively straight branches, the interlobular arteries and veins (W 304, 16.4) [example] extend up between the lobules of the cortex where they branch off into the intralobular arteries and, in turn, the afferent arterioles (click here to see an example) that supply the glomeruli within each lobule. Even though most of the RBCs have been washed out of the tissue in slide 210, the arcuate and interlobular vessels should still be identifiable by the smooth muscle in their walls (also, note that arcuate vessels are generally at the cortico-medullary boundary).
Efferent arterioles (do not worry about distinguishing between afferent vs. efferent arterioles), leaving the glomeruli, divide into peritubular capillaries which may be seen as small circular profiles amongst all the convoluted tubules. The majority of these capillaries then coalesce to enter the interlobular veins, allowing the blood to pass back to the general circulation. However, efferent arterioles from some glomeruli near the medulla (i.e., juxtamedullary glomeruli) provide the blood supply for the medulla. The multiple small vessels (arterioles that are more like dilated capillaries) arising from the efferent arterioles and descending into the medulla and the somewhat larger venules ascending from it are clustered to form the vasa recta, which you observed earlier in slide 204 as radiating reddish (or brownish) stripes in the medulla. The close association of arterioles and venules in the vasa recta provide counter-current exchange to help prevent loss of the high electrolyte concentration present in the inner medulla, necessary for the concentration of urine. Capillaries receiving blood from arterioles of the vasa recta are seen throughout the lower medulla. The venules of the vasa recta empty into arcuate or interlobular veins. Review the blood supply of the kidney. (UR2)
Other Potentially Useful Kidney Slides
Slide 203 (human kidney, tangential section, H&E) WebScope ImageScope. This section of kidney cortex was cut parallel to the surface of the kidney, and thus shows medullary rays in cross section (example). Observe such rays to see cross sections of straight proximal and distal tubules as well as collecting ducts. Also, you may have a more favorable view of maculae densae [example] in this slide.
Slide 209 (monkey kidney including pelvis, H&E) WebScope ImageScope. In this cross section of a monkey kidney, you will recognize cortex at the periphery and a medullary pyramid in the center. Review some of the features of kidney structure you saw in slides 204 & 210. Many of the tubules in the cortex are swollen, making it somewhat more difficult to distinguish proximal tubules from distal and collecting tubules. However, you may find structures in the medulla somewhat easier to interpret than those of slides 204 & 210.
Slide 205 (monkey kidney, vascular injection) WebScope ImageScope. An opaque red gelatin was injected through the renal artery of this kidney, filling many of the arteries and capillaries. Observe the distribution of blood vessels. It may require some insight to orient yourself on this section, since some of the cortex has been removed during preparation of the section. The vasa recta [example] are interesting here, since the descending arterioles are injected but the ascending venules did not receive the injection material and are full of red blood cells, which appear yellow.
Slide 206 (human fetal kidney, 200mm crown-rump length, H&E) WebScope ImageScope. Here you see a stage in kidney development. The kidney lobes (pyramids and their associated cortex) are particularly obvious at this stage of development, but eventually fuse to yield a smooth capsule with portions of each lobe forming the renal columns. You do not need to study this section in detail. The various components you have seen in previous slides are here, but in rudimentary form. One particular advantage to this section is that the RBCs are not washed out of the tissue and developing tubules in the medulla are separated quite nicely by connective tissue, so it is quite easy to discern vasa rectae, collecting tubules, and thick and thin portions of Henle's loops [example]. In this section you can also see an arcuate artery (which arches along the cortico-medullary boundary) arising from an interlobar artery [example].
II. URETER AND BLADDER (W pgs 326-7)
In these cross-sections of the ureter, note the transitional epithelium lining the lumen [example]. You studied the transitional epithelium (slide 19), in relaxed and stretched states, in a previous laboratory period dealing with epithelia. There are 2 to 3 layers of smooth muscle in the ureter. However, the smooth muscle is arranged in bundles that make the layers appear somewhat disorganized, thus it may be difficult for you to distinguish them. The connective tissue between the epithelium and the muscle is considered to be a lamina propria (there is no submucosa). There is an adventitia (connective tissue) outside the smooth muscle.
Compare the size of the bladder to that of the ureter and note that it is also lined with transitional epithelium [example]. You will remember that transitional epithelium is remarkable for its ability to stretch and yet maintain a strong barrier to diffusion of components in the urine. The muscular wall of the bladder, made up of bands of smooth muscle, provides for the expulsion of the urine during urination. Although three muscular layers are sometimes described in textbooks, they are usually rather difficult to distinguish in histological sections.
Electron Micrograph Wall Charts
#126 Survey view of kidney cortex WebScope ImageScope
Note that the brush border is present in proximal convoluted tubules, but not in distal convoluted tubules or in collecting ducts. There are numerous peritubular capillaries. This section of the renal corpuscle does not happen to include either the vascular or urinary poles. Note the abundant glomerular capillaries.
#127 Renal corpuscle WebScope ImageScope
Study the architecture of the glomerulus in this section of renal corpuscle. Review the organization and function of the glomerular endothelium, basement membrane and the podocytes. (The vascular pole is included in the plane of section, but not the urinary pole. The juxtaglomerular cells are secretory cells, derived from smooth muscle cells of the afferent arteriole. The macula densa shown here is cut in tangential section passing only through its wall, and does not include the lumen (the rest of the tubule is out of the plane of section). You can at least see the closely-packed nuclei that are characteristic of this modified portion of the distal tubule.
#128 Proximal convoluted tubule WebScope ImageScope
Note the brush border, composed of closely packed microvilli. Note the numerous mitochondria, in association with basal infoldings of the plasma membrane. Lateral cell membranes are not clearly visible because of extensive interdigitation of neighboring cells.
#129 Distal convoluted tubule WebScope ImageScope
Compare the distal convoluted tubule shown here with the proximal tubule seen in the previous micrograph. Note that, like the epithelial cells of the proximal convoluted tubule, these there are also abundant mitochondria (which you would expect since these cells are involved in the active transport of sodium). However, there are very few microvilli.
UR1: What are the three layers involved in glomerular filtration and how do they work? Answer
UR2: Explain the flow of blood through the kidney Answer
- The structure indicated is an: WebScope or ImageScope
- interlobar artery
- interlobar vein
- arcuate artery
- arcuate vein
- interlobular artery
- interlobular vein
- The structure indicated is: WebScope or ImageScope
- proximal convoluted tubule
- distal convoluted tubule
- proximal straight tubule
- distal straight tubule
- cortical collecting duct
- medullary collecting duct
- Which of the indicated tubules is generally the MOST permeable to water?WebScope or ImageScope
The cell indicated by the question mark is a:Click here to view image
- juxtaglomerular cell
- macula densa cell
- mesangial cell
- parietal epithelial cell of Bowman's capsule
- Compared to the osmolarity of blood plasma (~300 milliosmoles), the interstitial fluid within the area indicated is:WebScope or ImageScope
- hyper-osmotic (600 milliosmoles)
- VERY hyper-osmotic (1200 milliosmoles)
- Transitional epithelium:
- is found only in the ureters and bladder.
- is freely permeable to water.
- is freely permeable to salts.
- ALL of the above.
- NONE of the above.
- Which of the labeled ultrastructural features most significantly impedes the passage of negatively charged molecules?Click here to view image.