Plates 16-1 thru 16-6, pp. 342-351

Ch 19, Urinary System, pp. 332-347

Overview:

The goal of this lab is to examine the microscopic structure of the organs of the urinary system. For each organ particular emphasis should be placed on determining the classes of epithelium present and the specializations in each epithelium.

Diagram showing ultrastructure of epithelium along the rat nephron. Note indicated changes in apical and basal surfaces. All epithelial types have tight junctions (Weiss, L. Histology, 5th ed, 1983). See also Netter 321B, N323A, N325, and N326 for additional images of kidney structure.

Diagram summarizing transport functions along different nephron segments.  Not shown are thin limb exits of water (from descending tubules) and NaCl (from ascending tubules) down to hairpin turn.  Collecting duct shows ADH action; in diuresis, urine may be only 50 mosm/L.  Try to identify the arcuate and intralobular arteries & veins, efferent and afferent arterioles, and capillaries of peritubular and vasa recta systems.

Slide Descriptions

I. Kidney Paraffin Sections

204 kidney, human, H&E [Webscope] [Imagescope]
210 kidney monkey, H&E [Webscope] [Imagescope]
210 kidney monkey, PAS [Webscope] [Imagescope]

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 about 5 mm thick. Most of the remainder of the section is the medulla, forming renal pyramids (roughly triangular in appearance). The apex (tip) of the pyramid forms the papilla.

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.

A. Cortex

Examine the cortex of slide 204. 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

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 #204 [example] and #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, there may be small, white splits in the walls of these tubules, which should be ignored. In the cortex in slide 204, the straight portions of the proximal tubules (i.e. the descending thick limbs) #204 [example] are in the medullary rays, and have a similar histological appearance to proximal convoluted tubules. 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 [example].

Here and there among the proximal tubules in the cortical labyrinth you will also see distal convoluted tubules in #204 [example] and #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 #204 [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 #204 [example] and #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 #210 [example] (i.e. 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 [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

Examine the renal corpuscles found in the cortex, noting the numerous capillary loops of the glomerulus #210 [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 also belong to mesangial cells, but it is not possible to clearly distinguish these cells without the aid of an electron microscope.

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 [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 #204 [example] and #210 [example] will show a portion of this distal tubule with closely packed nuclei. This region of the distal tubule is the macula densa #210 [example] “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

Move to the medulla #210 [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:

1. An initial thick portion that represents the continuation of the straight proximal tubule from the medullary ray,
2. A thin descending portion that turns back toward the cortex as a thin ascending portion that is continuous with
3. 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 #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 #210 [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 #210 [example] (somewhat damaged in #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

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 #204 [example], sizeable vessels passing along the boundary between the cortex and medulla. From the arcuate arteries, relatively straight branches, the interlobular arteries and veins #204 [example] extend up between the lobules of the cortex where they branch off into intralobular arteries and, in turn, the afferent arterioles #210 [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.

II. Kidney Thin Section

Webslide 0049_V:  Monkey kidney, 1.5 µm, H&E [ImageScope] [WebScope]              

Whereas the slides you've seens thus far are paraffin sections (5-10 um thick), this is a thin section (1.5 um thick) showing portions of both medulla and cortex and therefore the best slide for detail of cortical labyrinth structures and tubules.  Examine it with all powers.

In the cortex:
Proximal versus distal convoluted tubules as seen in this slide:

Proximal convoluted tubules [example]: dark staining, low but definite brush border despite occlusion of lumen with stained proteinaceous material.

Distal convoluted tubules [example (in center of field)]: less numerous, paler staining, more open lumen, often smaller diameter in comparison to PCTs.

Renal corpuscles [example]: look for one showing urinary and/or vascular pole; note Bowman’s capsule and glomerulus; look for cross sections of glomerular capillaries with endothelial cell nuclei projecting into the blood spaces; locate podocyte nuclei projecting into urinary space, and perhaps mesangial cells which lie between capillaries.  Look for serpentine clefts of urinary space between capillaries. Note: A macula densa at the vascular pole of a renal corpuscle is marked in an arrow.

Arteries and veins [example (artery, cross sect.)] [example (artery, long. sect.)] [example (vein, long. sect.)]: especially near medulla; some of the very large blood spaces are probably over-expanded.

Peritubular capillaries: look for these between convoluted tubules in the cortical labyrinth. These are the “other” capillaries of the cortex; they are in series with glomerular capillaries, are fed by the efferent arterioles, and they empty into veins, NOT into arterioles.

In the medulla:

Vasa recta [example]: filled with blood.

Thin segments of Henle’s loop [example (squamous-lined tubes NOT filled with blood)]: thin-walled, partially collapsed vessels among, but distinct from vasa recta; these are rare because the section does not go very deep into the medulla. When seen, nuclei of thin-limb simple squamous cells may bulge into lumen more prominently than nuclei of endothelium in adjacent vasa recta.

Ascending thick limb [example]: in the medulla, cuboidal epithelium, darker staining than collecting tubules.

Collecting ducts [example]: large pale cells of simple cuboidal epithelium with clearly visible borders between cells.

III. Ureter

Slide UMich 211 ureter, human, H&E [Webscope] [Imagescope]

In this cross-section of the ureter, note the transitional epithelium lining the lumen [example]. Also, this section is lower in the ureter where 3 layers of the muscularis can be seen: inner longitudinal, middle circular, and outer longitudinal. 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.

IV. Bladder

Webslide 0098_G: Urinary Bladder, monkey, H&E
[WebSlide ActiveX] [WebSlide Java] [Aperio ImageScope] [Aperio WebScope]

Note the thick muscular wall [example] and the lining epithelium.  At high magnification examine the layer of epithelial cells next to the lumen [example].  Large surface cells bulge into the lumen, showing scalloped edges (like a choppy seascape) indicative of the rough texture of this surface. You may notice the presence of granules and vesicles within the cytoplasm of the lining cells. The granules are likely aggregates of membrane-bound uroplakin, which is one of the components inserted onto the surface to make the epithelium impermeable to water and ions in the urine. The vesicles are reserves of membrane that can be rapidly inserted onto the surface when the cells need to expand as the bladder fills.