URINARY SYSTEM

The urinary system consists of the two kidneys and the excretory passages, which convey urine from the kidneys to the exterior of the body. Excretory passages include the minor calyces, major calyces, renal pelvis (one for each kidney), the two ureters, the urinary bladder, and the urethra. Urine formed in either kidney is emptied into the minor calyces and then passes via the major calyces, renal pelvis, and ureter to the urinary bladder where it is stored until conducted by the urethra to the exterior of the body.

Kidney

Before examining the sections of kidney, study the text illustrations (Figure 96 below) and become familiar with the large subdivisions of the kidney as seen in a radial section.

Figure 96: Schematic representation depicting the general organization of the kidney and a juxtamedullary nephrons with its collecting duct. Taken from Junqueira and Carneiro, Basic Histology, a text and atlas, p. 384, Figure 19-1.

Examine slides B89 and B90, kidney, human (H&E). Observe the capsule (if present), cortex, renal corpuscles, and medulla, which has no renal corpuscles. Using slide B90, locate an area in the cortex where tubules run parallel to one another and are cut longitudinally. This is a pars radiata or medullary ray. On either side is a pars convoluta, which contains renal corpuscles and coiled tubules. Under higher magnification, study the proximal and distal convoluted tubules within a pars convoluta and note their distinguishing features. Compare with Figure 97 below. The comparison is more easily made on slide B89. A proximal convoluted tubule (PCT) is more than twice as long as a distal convoluted tubule (DCT); consequently, the vast majority of tubules are proximal. The diameter of the DCT is much less than that of the PCT, although the luminal diameter of the two tubules may be about the same. Fewer nuclei appear in a cross section of a PCT than a DCT. The luminal surface of a DCT is sharper than that of a PCT, which has an uneven star-shaped appearance due to the presence of a brush border (to be studied in slide B94, PASH). The epithelium of a PCT is darker staining and usually taller or more columnar than that of a DCT. Cells of the DCT are cuboidal. Those cells of the PCT are heavily granular due to cytoplasmic lysosomes, and have indistinct cell boundaries. A precipitate is present in the lumina of the proximal tubules in slides B89 and B90, but this is not a constant feature and identification of proximal tubules on this basis is tenuous. The distal tubules are usually more lightly stained and are free of lysosomes.

Figure 97: Schematic diagram of the cellular Ultrastructure of the nephrons. Taken from Junqueira and Carneiro, Basic Histology, a text and atlas, p. 393, Figure 19-16.

Examine slide B90, kidney, human, (H&E). Note how the glomerulus is suspended in the urinary space. It attaches at the vascular pole where the afferent and efferent arterioles enter and leave the glomerulus. Note that the glomerulus does not have a smooth contour but that the surface is deeply indented so as to divide the glomerulus into lobules.

Study slide B91, kidney, human, (H&E). This slide is very similar to slide B90. Identify the various structures found within the cortex. A renal papilla is evident on this slide.

Study slide B92, kidney, human, (PAS). Study the structure of a Bowman’s capsule. The parietal layer consists of a basement membrane and a layer of epithelial cells. Both are continuous with the neck of the proximal tubule. The visceral layer lies on the glomerulus and is continuous with the parietal layer at the vascular pole. It consists of a basement membrane (fused with the basement membrane of the glomerulus) and a layer of visceral epithelial cells, termed podocytes because with the electron microscope they are seen to have foot processes (pedicels). Examine a number of capsules until one is found which is sectioned through the vascular and urinary poles. The brush borders of the proximal tubules are not as well demonstrated. Maculae densae are easily found.

Study slide B90, kidney, human, (H&E). Observe the medullary rays. The rays contain straight portions of proximal and distal tubules and collecting ducts. The straight portion of a proximal tubule is similar to a DCT. A collecting tubule, in contrast to a proximal tubule, lacks a brush border and differs from a distal tubule in that it is usually larger and has distinct cell boundaries.

Study slide B92, kidney, human, (PAS) and slide B94, kidney, rabbit, (PAS). Identify the medullary rays and observe the PAS-positive basement membrane that surrounds each kidney tubule. In slide B92, locate a pars convoluta and examine the proximal and distal convoluted tubules. Search for an area with the low power objective where a DCT makes contact with the vascular pole of its renal corpuscle. With the high dry objective, note that the nuclei of the DCT cells are very close together at this point, and in a palisade arrangement; this is called the macula densa, and is the part of the DCT in contact with the afferent glomerular arteriole (see Figure 98 below). The cells in the afferent glomerular arteriole, which contact the macula densa, are called juxtaglomerular cells (JG cells). The electron microscope shows that these cells lack myofibrils, in contrast to the typical smooth muscle cells of the arteriole, but they contain secretory granules, granular endoplasmic reticulum, and a Golgi apparatus. Thus, these cells have the characteristics of secretory cells; they secrete renin and erythropoietin. The JG cells and the macula densa constitute a juxtaglomerular apparatus (JGA). After finding a JGA in slide B92 try and locate another one in slide B94.

Figure 98: Left photo, micrograph of Juxtaglomerular apparatus. DCT = distal convoluted tubule, MD = macula densa, L = extraglomerular cells, J = Juxtaglomerular cells, AA = afferent arteriole. Right photo, schematic diagram of the Juxtaglomerular apparatus for comparison. Both taken from, Wheater’s Functional Histology, a text and colour atlas, pp. 302-303, Figure 16-18.

Study slide B91, kidney, human (H&E). Continue to examine the tubules in the medulla. The tubules lined by columnar cells with pale cytoplasm and distinct cell boundaries are the straight collecting tubules which unite to form the large papillary ducts (ducts of Bellini) that open into the minor calyces at the inner zone of the medulla. The smallest tubules are the thin segments of the loops of Henle. They resemble small veins in that they are about 15 mm in diameter and have a lining of simple squamous epithelial cells whose nuclei are far apart and bulge into the lumen. Thin segments, straight collecting tubules, and papillary ducts are present in the inner zone of the medulla (near the minor calyces). In contrast, the outer zone of the medulla lacks papillary ducts, but contains thick segments of the loops of Henle in addition to thin segments and straight collecting tubules. There are two types of thick segments: (1) thick, descending limbs of the loops of Henle (representing the straight portions of the proximal tubules), and (2) the thick, ascending limbs of the loops of Henle (representing the straight portions of the distal tubules).

Study slide B93, kidney, monkey, (H&E). This section is useful for examining the overall architecture of the kidney.

Examine slide BB-65, kidney, dog, (Carmine). This tissue was prepared by injecting into the renal artery a warm mixture of gelatin stained with carmine, thus filling the vascular bed. Study and compare Figure 99 below with the slide under your scope. The large arcuate arteries at the corticomedullary junction can be seen. Arcuate arteries, like the interlobar arteries, are end arteries in that they do not anastomose with each other.

Figure 99: Micrograph of the vascular supply to the glomerulus. G = glomerulus, BS = Bowman’s space, AA = afferent arteriole, ES = efferent arteriole, RT = renal tubules and IA = interlobular artery. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 293, Figure 16.10.

Interlobular arteries arise from arcuate arteries and can be seen in slide BB-65. They lie within the pars convoluta of the cortex, about midway between two adjacent medullary rays. The area between two interlobular arteries is a kidney lobule. It includes a medullary ray (pars radiata) and half of each adjoining pars convoluta. All the nephrons of a lobule drain into a common collecting tubule. Each interlobular artery gives off a number of afferent glomerular arterioles before it ends in a capillary bed beneath the capsule.

Each afferent glomerular arteriole supplies a glomerulus. The smooth muscle in the wall of the vessel serves as a stopcock to influence the rate of blood flow to the glomerulus.

A glomerulus consists of 20-40 anastomosing capillary loops inside Bowman's capsule. As an afferent arteriole enters Bowman's capsule, it forms 5-8 primary branches with each branch forming a number of capillary loops called a lobule. Anastomoses occur between capillary loops within a lobule as well as between capillary loops of adjacent lobules. The capillaries of the glomerulus converge to form the efferent glomerular arteriole.

The efferent glomerular arterioles in the outer part of the cortex divide into cortical capillaries, which surround the cortical tubules and form a peritubular plexus. These blood vessels empty into the cortical venules, which drain into the interlobular veins. The efferent arterioles of the more deeply situated glomeruli are of larger caliber than those of the outer cortex. They pass into the medulla (some as far as the papillae of the pyramids) as the vasa rectae. The vasa rectae are thin-walled blood vessels, which make hairpin loops deep in the medulla. The ascending limb of each loop returns to the corticomedullary junction to enter an arcuate vein. The vasa rectae accompany the loops of Henle and are important in the countercurrent mechanism. These can all be observed in slide BB-65.

Arcuate veins can be seen with the arcuate arteries at the corticomedullary junction. They return blood to the interlobar veins, which, in turn, empty into the renal vein, a tributary of the inferior vena cava.

Study slide B97, kidney and adrenal, human fetus, (H&E). Lobation is quite evident in this section. Interlobar and arcuate vessels are present, as is a section of the ureter.

Study, slide B89, kidney, human, (H&E). Note that the quantity of interstitial connective tissue (interstitium or stroma) in the kidney is very small. The bulk of the kidney consists of parenchyma (functional tissue), namely the tubules, and only a small amount of stroma is present. A few small collagenous fibers surround the capsules of Bowman and the larger papillary ducts. Bundles of collagenous fibers are found only in the adventitia of the larger blood vessels. Elastic fibers are absent except in the walls of the blood vessels. Silver stains show a diffuse network of reticular fibers between the kidney tubules. Collagenous fibers in the kidneys may increase considerably in old age, and they are believed to develop from the reticular fibers.

Drawing of renal corpuscle and kidney tubules

Study several renal corpuscles in slide B89 and prepare an outline drawing which shows a renal corpuscle, its urinary and vascular poles, the juxtaglomerular complex, and the lobulations of the glomerulus. Include several sections of proximal and distal convoluted tubules. Use the text, if necessary, for filling in details and for labeling.

Table 5: Summary Table comparing the epithelial structure in the different parts of the renal tubule. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 307, Figure 16.21b.

Ureter

The two ureters are the part of the excretory passageway, which conduct urine from the kidneys to the bladder.

Study slide C1, ureter, human, (H&E). Using the low power objective, identify the three tunics (mucosa, muscularis, and fibrosa), which form the wall. The mucosa or mucous membrane, consisting of epithelium and lamina propria, is highly folded and has a number of outpockets. The narrow, star-shaped lumen is lined by transitional epithelium, which is 5-6 cell layers thick. Under higher magnification, note that the surface layer of cells is larger and darker staining than the deeper cell layers. Cells of the basal layer are cuboidal to columnar in shape. The transitional epithelium of the ureter and of the other excretory passageways rests upon a very thin basement membrane, the thinnest in the body. It was questioned whether transitional epithelium had a basement membrane until one was clearly shown in electron micrographs.

The lamina propria has a rather dense appearance and contains an abundance of collagenous fibers. Numerous elastic fibers are also present; making the lamina propria a stretchable fibroelastic connective tissue, but these fibers cannot be readily visualized without special stains (e.g., Verhoeff). Besides connective tissue fibers, small veins tending to form a plexus, and various types of connective tissue cells are present in the lamina propria. Glands are absent. Note that the lamina propria is of a somewhat looser texture in its deeper portion where the connective tissue fibers intermingle with the bundles of smooth muscle.

The regular longitudinal folds of the mucosa, show in slide C1 in cross sectional profile, are characteristic of the ureter.

The muscularis of the upper two-thirds of the ureter is divided into inner longitudinal and outer circular layers. This arrangement of smooth muscle is opposite that of the intestines where inner circular and outer longitudinal layers are present. The muscularis of the lower one-third of the ureter has inner longitudinal, middle circular, and outer longitudinal layers. Unlike the situation in the gastrointestinal tract, the muscle layers are neither compact nor continuous. Each layer is formed by bundles of smooth muscle separated from each other by large amounts of connective tissue. The smooth muscle contributes to the flow of urine by peristaltic contraction.

The fibrosa lies external to the muscularis. It contains loose connective tissue, blood vessels, lymphatics, and nerves.

Study slide C2, ureter, human, (H&E). This section is similar to C1, although the mucosal folds are not quite as regular.

Study slide B97, kidney and adrenal, human fetus, (H&E).

Study slide C3, ureter, monkey, plastic, (H&E). The transitional epithelium is exceptionally well preserved in this section. Note the occurrence of binucleated cells in the superficial layer.

Urinary bladder

The urinary bladder is an expandable, hollow, muscular sac, which receives urine from the two ureters and temporarily stores it until discharged via the urethra. Like the ureter, the bladder has three layers (mucosa, muscularis, and fibrosa or serosa) with each layer being much thicker than the corresponding layer of the ureter.

Study slide C4, urinary bladder, monkey, (H&E). This section shows the components of the bladder wall. The mucosa with its lining of transitional epithelium and underlying lamina propria is rather thick. The epithelium has 6-8 cell layers, a characteristic of the relaxed (contracted) bladder. The surface cells, like those frequently seen in the liver, contain multiple amounts of DNA and demonstrate polyploidy. A thin cuticular border can be seen at the luminal surface of some cells. The border, as shown by electron microscopy, is produced by tonofilaments, which tend to congregate in the apical cytoplasm. As part of the cytoskeleton, they provide cellular rigidity, but perhaps a more important function is that they may prevent resorption of the hypertonic urine. The cells immediately beneath the surface cells are pear-shaped and their apices fit into the facet-like indentations on the underside of the surface cells. The cells between the surface and basal cell layers make up the intermediate cell layer. They are smaller, more elongated, and more irregularly shaped than the surface cells. The basal cell layer is like that in other types of stratified epithelia. It undergoes mitosis to replenish the cells that are lost at the free surface.

The lamina propria, a layer of fibroelastic connective tissue is separated from the epithelium by an indistinct basement membrane, which is rather dense in its superficial portion but becomes looser in texture as the muscularis is approached. I t is not unusual to find lymphocytes in abundance in the denser, superficial portion, and some of them may be seen migrating through the epithelium. A submucosa is not present, although as with the ureter, the looser connective tissue near the muscularis is sometimes referred to as a submucosa. Glands are absent. The mucous surface coat that is seen with the electron microscope on the free surface of transitional epithelium is believed to be produced by the surface cells.

The muscularis is well developed and, like the epithelium, shows differences in thickness with the expansion or contraction of the bladder. Three layers of muscle are described (except in the trigone): an inner longitudinal, a middle circular, and an outer longitudinal, but the interlacing of bundles of smooth muscle makes it difficult to differentiate between the layers. First observe the different bundles of smooth muscle, their direction, and their distribution, and then attempt to group them into layers. Note the loose connective tissue, blood vessels, and nerves between the smooth muscle bundles. It is not unusual to find groups of neurons (small ganglia) in the connective tissue, and these are indicative of the innervation of the smooth muscle by the autonomic nervous system.

The external covering of the bladder is a fibrosa except on the superior surface where peritoneum is present. Thus the external covering on the superior surface is a serosa. The outer connective tissue layer of other viscera is also called a serosa if it is covered by the mesothelium of the peritoneum. From the superior surface of the bladder, it will show the serosa with its mesothelium (simple squamous epithelium), fat, loose connective tissue, blood vessels, and nerves.

Study slide C5, urinary bladder, collapsed, human, (H&E). This section shows the wall of the bladder as it appears when empty or relaxed. Epithelial preservation is spotty; however, in places multiple layers of cells are apparent.

Study slide C31, prostate and bladder, human, (H&E). The section of bladder is another example of being collapsed.

Study slide C6, urinary bladder, expanded, (H&E). This section shows the wall of the bladder as it appears when “full” or expanded. Note the stretched appearance of the smooth muscle and the reduced thickness of the transitional epithelium.

Study slide C9, urethra, cross section, female human, (H&E). The irregular profile of the urethral lumen is seen with the epithelium projecting into “lacunae”. Glands of Littre may or may not be present. If so, they usually open into the lacunae. A portion of the anterior wall of the vagina is present along one side of the section.

Study slide C10, urethra, human female, (H&E). The epithelium is mixed at this level, partly stratified squamous and partly stratified columnar. Beneath the epithelium are numerous blood-filled venous sinuses. Further out is a prominent band of skeletal muscle surrounding about 75% of the urethra.

Study slide BB-67, urethra, cross section, female, (H&E). The urethral lumen is C-shaped and is lined with transitional epithelium at this level. Note the large vascular spaces (sinuses), which are similar to those of erectile tissue in the male.

Study slide BB-75, urethra, male, (H&E). Lacunae with mucous glands (Littre) are well demonstrated.