|Atlas||Wheater's, pgs. 302-27, Urinary System|
|Text||Ross and Pawlina, Chapter 20, Urinary System|
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.
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 204 kidney H&E Webscope Imagescope about 5 mm thick. Most of the remainder of the section is the medulla 204 kidney H&E Webscope Imagescope, forming renal pyramids (roughly triangular in appearance). The apex (tip) of the pyramid forms the papilla 204 kidney H&E Webscope Imagescope.
The short red strips, which you may see in the medulla in slide 204 are "vasa recta" #204 Webscope ImageScope (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. You will recognize medullary rays (or pars radiata) #204 Webscope ImageScope, 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) #204 Webscope ImageScope, contains renal corpuscles and the convoluted portions of the tubules.
Identify the three general types of tubules that occur in the cortical labyrinth and medullary rays of the cortex:
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 #204 Webscope ImageScope and #210 Webscope ImageScope 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 #204 Webscope ImageScope 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 #204 Webscope ImageScope.
Here and there among the proximal tubules in the cortical labyrinth you will also see distal convoluted tubules in #204 Webscope ImageScope and #210 Webscope ImageScope. 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 #204 Webscope ImageScope 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 #204 Webscope ImageScope and #210 Webscope ImageScope. 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 #210 Webscope ImageScope (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 #204 Webscope ImageScope in these small vessels. The kidney in slide 210 was perfusion-fixed and, therefore, the capillaries are devoid of red blood cells.
Examine the renal corpuscles found in the cortex, noting the numerous capillary loops of the glomerulus #210 Webscope ImageScope. 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? answerThere are three layers that make up the filtering system of the glomerulus. These are the capillary endothelium, the glomerular basement membrane (made of the fused basement membranes of the endothelial cells and podocytes) and the podocyte layer. Renal capillaries are fenestrated capillaries without diaphragms. They are covered with podocalyxin, a negatively charged molecule. Their negative charge helps prevent negatively charged molecules pass through the filter. Between the podocytes and fenestrated capillary endothelial cells of the kidney there is a fused basal lamina made of two lamina rara and a single lamina densa (the glomerular basement membrane). Embedded in the lamina densa are molecules of perlecan, which is made up of negatively charged heparin sulfate chains. The lamina rarae are particularly rich in such polyanions, so they contribute significantly to this charge exclusion filter which prevents negatively charged molecules from passing through. The collagen IV and laminin meshwork of the lamina densa serves as a size barrier, helping to prevent the passage of proteins through the filter. The secondary processes of the podocyte interdigitate to form filtration slits with diaphragms between them. They are also covered with podocalyxin that helps keep the processes apart and also serves as a last effort to prevent the filtration of negatively charged molecules. They also prevent large molecules (like proteins) from coming through. Their main purpose, however, is to regulate water flow. (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 #210 Webscope ImageScope. Look around under low power to find glomeruli sectioned through the vascular pole. Near the vascular pole will be the distal tubular part of the same nephron. Some sections in #204 Webscope ImageScope and #210 Webscope ImageScope will show a portion of this distal tubule with unusually closely packed nuclei. This region is the macula densa which is located at the end of the ascending straight tubule where the distal convoluted tubule begins #210 Webscope ImageScope . The macula densa together with the juxtaglomerular cells and the extraglomerular mesangial cells (lacis cells) form the "juxtaglomerular apparatus". Juxtaglomerular cells are specialized smooth muscle cells found in the wall of the afferent (and to some extend the efferent) arteriole which secrete renin. You cannot distinguish juxtaglomerular cells in these preparations (but you could detect them by immunological techniques, e.g. immunostaining for renin).
Move to the medulla #210 Webscope ImageScope, 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:
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 #204 Webscope ImageScope). 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 #210 Webscope ImageScope). 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 #210 Webscope ImageScope (somewhat damaged in #204 Webscope ImageScope) 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.
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 interlobar arteries and veins #204 Webscope ImageScope, sizeable vessels passing along the lateral sides of the medullary pyramid. Arcuate arteries and veins follow the base of the medullary pyramid along the boundary between the renal medulla and the renal cortex. From the arcuate arteries, relatively straight branches, the interlobular arteries and vein #204 Webscope ImageScope extend up between the lobules of the cortex where they branch off into the intralobular arteries and, in turn, the afferent arterioles #210 Webscope ImageScopethat 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 lining the base of the medullary pyramid along 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. Explain the flow of blood through the kidney answerBlood enters the kidney through the large renal arteries. At the hilum, the renal arteries branch and become interlobar arteries. Interlobar arteries travel through the medulla to the corticomedullary junction where the arteries branch into arcuate arteries that run along the corticomedullary junction. The arcuate arteries further branch and become interlobular arteries that run through the renal cortex. From the interlobular arteries come afferent arterioles that become the glomerulus. Exiting from the glomerulus is the efferent arteriole. After leaving glomeruli in the cortical region, the efferent arteriole leads to the peritubular capillary network. Efferent arterioles of the juxtamedullary glomeruli become the vasa rectae, which can be seen in the medulla. The vasa rectae and peritubular capillary network drain directly into interlobular veins. Peritubular capillaries drain into stellate veins and then into interlobular veins. From there blood travels to the arcuate veins, interlobar veins and finally leaves through the renal vein.
This section of kidney cortex was cut parallel to the surface of the kidney, and thus shows medullary rays in cross section #210 Webscope ImageScope. 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 #203N Webscope ImageScope in this slide.
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.
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 #205 Webscope ImageScope 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.
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 #206 Webscope ImageScope. In this section you can also see an arcuate artery (which arches along the cortico-medullary boundary) arising from an interlobar artery #206 Webscope ImageScope.
In these cross-sections of the ureter, note the transitional epithelium lining the lumen #211 Webscope ImageScope. 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 #211 Webscope ImageScope. 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.