URINARY SYSTEM Part I (Top and Bottom Portions-Not the Loop)

URINARY SYSTEM Part I (Top and Bottom Portions-not the loop) Roger Bick, PhD, MMEd, FAHA.

Reading: Gartner and Hiatt, pp. 337-350; Gartner, Hiatt, Strum, pp 275-281 and 284-285, 287- 293

Learning Objectives:  Name components of functioning renal parenchyma versus passive collecting system.  Be able to describe the flow of blood through the kidneys from the renal artery back to the renal vein via either a superficial or a juxtamedullary glomerulus.  Recognize the main divisions of the nephron with light microscopy and know their main functions.  Name the components of the juxtaglomerular apparatus.  List the components of the glomerular filtration barrier.  Be able to identify the following components of the renal corpuscle with light microscopy: parietal/visceral epithelium, mesangium, capillaries, urinary pole, vascular pole, and Bowman's capsule.  Be able to identify the following components of the renal corpuscle by electron microscopy: epithelial, endothelial, and mesangial cells; mesangium; basement membrane.  Understand the zones of the kidney and the locations of various parts of the nephron within those zones.

Key Words: Cortex, medulla, nephron, transitional epithelium, ureter, Renal corpuscle, glomerular filtration barrier and bladder.

GROSS FEATURES A. KIDNEY (1) Location- The kidneys are located in the retroperitoneal space with the upper pole at the level of T12 and the lower at L3. (2) Fetal Features - In fetal life, the kidneys are composed of 7-18 pyramid-shaped lobes (bases externally oriented, apices pointing inward) giving them a lumpy external appearance. The demarcation between the lobes is obscured in time, so that the kidney ultimately has a smooth appearance. It is important to be aware of the early lobar composition of the kidneys to understand the internal architecture and nomenclature. (3) Adult Features -Each kidney weighs 120 to 170 gm and is 11-12 cm in length. a. Functional parenchyma (1) Cortex: This occupies the outermost rim of tissue grossly, being about 1 cm in thickness. Large vessels can be seen and delineate the cortex from the next zone inward, the medulla. (2) Medulla: This zone is medial to the cortex and contains 7-18 conical masses called renal pyramids. The base of each pyramid is at the corticomedullary boundary, while the apex of each extends inward to the renal pelvis, forming a nipple-like protrusion called a papilla. Sometimes several pyramids terminate in a common papilla, so that there are fewer papillae than there are pyramids. Urine is released from small holes in the papillae into a passive collecting system. Extensions of the cortex between adjacent pyramids are called renal columns (of Bertin). Each pyramid and the surrounding cortical tissue in the renal column comprise a lobe, the total number corresponding to the number of lobes in the fetal kidney. b. Intrarenal (In the kidney) Collecting System

1 (1) Minor Calyx: Each minor calyx is a cup-like structure that surrounds 1-2 papillae and collects the released urine. (2) Major Calyx: The minor calyces funnel into 2-3 larger major calyces. (3) Pelvis: The major calyces funnel into this large atrium-like area deep within the kidney.

c. EXTRARENAL COLLECTING SYSTEM (1) Ureter: The pelvis gradually narrows into this tube-like conduit which runs from the kidney to the bladder. The proximal part of the ureter begins in the renal hilum, defined as the medial aspect of the kidney where the renal artery and vein, ureter, and fat are found. (2) Bladder (3) Urethra

ARTERIAL SYSTEM Blood enters the kidney at the hilus via a renal artery, branching into interlobar arteries perpendicular to the surface, between the renal lobes. From these, arcuate arteries branch off at right angles, running parallel to the surface between the cortex and medulla. (They are very useful landmarks since they define the border between cortex and medulla.) Smaller branches are then given off into the cortex, again running perpendicular to the surface, called interlobular arteries. Circulation distal to these arteries will be considered with other histologic features.

2 INNERVATION The kidney is richly supplied with sympathetic nerves that follow the course of the renal arteries and arterioles and innervate the smooth muscle in their walls to contract. Strong sympathetic stimulation constricts these vessels, increasing their resistance and decreasing renal blood flow. Sympathetic nerves are also found on tubules and can directly cause increases in sodium absorption.

HISTOLOGIC FEATURES A. COLLECTING SYSTEM 1. Minor Calyx to Bladder The calyces, renal pelvis, ureter and bladder have the same basic histologic structure. a. Mucosa (1) Epithelium: Transitional; increase from 3 cell layers deep at the minor calyx to 6 in the bladder (2) Lamina Propria: Loose to dense connective tissue. NOTE: More than 90% of urinary bladder tumors originate in the epithelial lining. b. Muscularis Begins with the minor calyces, and consists of 2 layers of smooth muscle, with the innermost being longitudinal and the one external to this being circular. As the ureter approaches the bladder, a third outer longitudinal layer is added, which is constant throughout the bladder. c. Adventitia or Serosa Only the dome of the bladder is covered by serosa (peritoneum). The wall of the rest of the bladder and the ureter blend into the surrounding tissues without a defined serosa.

2. Transitional Epithelium Dynamics This epithelium is unique in the necessity for it to be able to expand and contract rapidly. This is accomplished by the presence in the apical cytoplasm of discoid vesicles, the membranes of which serve as reservoirs for apical membrane expansion as follows. When the bladder is empty (A), the apical cytoplasm of the innermost cells is filled with discoid vesicles underneath the plasmalemma. As the bladder fills and the epithelium stretches (B), these vesicles fuse with the apical membranes, providing increased surface area.

Figure 19—27. Compare the structure of the transitional epithelium when the urinary bladder is empty (A) or full (B). When the bladder is full, the capacity of epithelial cells to slide upon one another reduces the thickness of the epithelium. As a result, the interior surface of the bladder increases. In B, note the thin strands of collagen fibers separating bundles of smooth muscle cells. PSH stain. Medium magnification

3 3. Urethra Near the bladder, the urethra has transitional epithelium. Adjacent to the "external world", the epithelium is stratified squamous. Between the zones, the epithelium may be stratified columnar or pseudostratified columnar. The female urethra is mostly lined with stratified squamous epithelium. Glands may empty into the lumen of the urethra of the male or female. There are abundant elastic fibers in the lamina propria. The muscularis proximally consists of inner longitudinal and outer circular smooth muscle. At the urogenital diaphragm, voluntary striated muscle fibers are mixed with the outer layer of smooth muscle. Beyond this, the muscularis begins to disappear and ceases to exist as one of the layers. There is no serosa. In the female, the connective tissue, of the adventitia fuses with the vaginal canal. More details regarding the male urethra will be provided in a later lecture.

B. THE NEPHRON - Main Divisions This is the functional unit of the kidney, with 1,300,000 comprising each kidney. Each nephron is further subdivided into areas that differ structurally and functionally. A relatively small amount of connective tissue containing abundant capillaries, some cells and matrix separates the tubules. This connective tissue space is called the interstitium. 1. Renal corpuscle a. Glomerulus b. Bowman's capsule c. Bowman's space 2. Proximal tubule a. Proximal convoluted tubules b. Thick descending limb 3. Thin limb a. Thin descending limb b. Thin ascending limb 4. Distal tubule a. Distal straight tubule (thick ascending limb) b. Distal convoluted tubule 5. Collecting tubule/duct

NOTE: Loop of Henle - Includes the straight, or last portion of the proximal tubule, the thin limb, and the distal straight tubule or thick ascending limb that lie in the medulla.

THE RENAL CORPUSCLE (Malpighian Corpuscle) 1. Structure This is a complex structure which absolutely requires both a two and three dimensional conceptualization for complete understanding. The adult glomeruli measure approximately 200 µm in diameter. a. Vascular Component - The Glomerulus (plural: glomeruli) We last left circulation at the level of the small cortical interlobular arteries. Branching off these are afferent arterioles. Each afferent arteriole subdivides into a brain-like convolution of capillaries. Each ball of capillaries constitutes a glomerulus (glomerular capillaries; glomerular tuft). The capillaries merge back into a single efferent arteriole that carries blood away. The fate of the circulation from this point will be continued later. At this point it will be noted that some cells within the walls of the afferent and efferent arterioles, in addition to being of smooth muscle origin typical of other arterioles, also contain granules of renin. They are called juxtaglomerular cells or granular cells. The significance of this feature will also be subsequently discussed. The glomerular capillaries are characterized by an inner simple squamous fenestrated endothelium in which there are no diaphragms across the fenestrae. This layer is thus very leaky and presents little barrier to the movement of plasma. The endothelial cells rest on a glomerular basement membrane (GBM), which is an amorphous extracellular, gel-like layer. It is thought to be the major barrier to the movement of plasma. b. Glomerular basement membrane (GBM) Major components of the GBM include Type IV collagen and laminin which both contribute to adhesion of the epithelia to the GBM, and proteoglycans such as heparan sulfate, which contribute an overall negative charge to the GBM. The GBM is not totally homogeneous, but can be seen with transmission electron microscopy to have the following layers:

Lamina Rara Interna: An electron-lucid layer directly under the endothelium. Lamina Densa: An internal electron-dense layer.

Lamina Rara Externa: Electron-lucid layer found under the visceral epithelium. The foot processes or pedicels of the visceral epithelium or podocytes (see below) rest directly on the GBM. Like the endothelium, the visceral epithelium contributes to the formation of this extracellular layer. The GBM is thus sandwiched tightly between the

5 visceral epithelium on one side and the glomerular endothelium on the other side. The GBM, interestingly, represents a fusion of the basal laminae, specifically the laminae densa, of two epithelia, a unique arrangement in the body. (Review the portion of the connective tissue lecture on basement membranes to better visualize this recalling that, since there is no underlying connective tissue, there will not be a lamina reticularis). c. Epithelial Component (1) Visceral Epithelium: This epithelium closely invests the glomerular capillaries. It is unique in the body in that it consists of octopus-like cells called podocytes (visceral epithelial cells). The "arms" of these cells encircle the capillaries and are called foot processes or pedicels. The foot processes or pedicels rest directly on the GBM. The pedicels of neighboring cells interdigitate, but do not actually touch. They are separated by narrow spaces called filtration slits. Each slit is bridged by a thin membrane called a filtration slit diaphragm, which in turn is penetrated by a zipper-like array of rectangular pores. It is through these pores that plasma is able to pass through the visceral epithelial layer, and they provide little resistance to flow. Since each pore is slightly smaller than an albumin molecule, macromolecules of this size and larger are normally retained within the vasculature and do not filter.

(2) Parietal Epithelium; Parietal Layer of Bowman's Capsule: The visceral epithelium coats the glomerular capillaries, but where they merge with the arterioles, called the vascular pole, this layer reflects away and transitions immediately into a simple squamous epithelium. It forms a thin veil around the glomerular tuft and is separated from the tuft by a space, called Bowman's space. On the opposite side of the tuft from the vascular pole the parietal layer transitions into a tall columnar type of epithelium that forms the beginning of the tubular system. This is the urinary pole of the renal corpuscle. In summary, the visceral and parietal epithelial layers both compose the Bowman's capsule, but have very different appearances and functions and are separated by Bowman's space (urinary space). d. The Glomerular Mesangium Connective tissue cells are found between the afferent and efferent arterioles - Extraglomerular mesangial cells, and the bases of the capillary loops - Intraglomerular mesangial cells. They are surrounded by an amorphous matrix. Mesangial cells and matrix are collectively referred to as the mesangium. Aside from a supportive role, the function of the mesangium is unclear but under intensive investigation. It is known that the cells contain myosin and have contractile properties. They also play a role in uptake and elimination of small particles and antigen-antibody complexes. Other functions are speculated.

6 e. The Glomerular Filtration Barrier Plasma must filter out of the glomerular capillaries through various layers of the renal corpuscle into Bowman's space, where it is then called the filtrate. The layers traversed in this passage are: (1) Glomerular capillary endothelium (2) Glomerular basement membrane Lamina rara interna Lamina densa Lamina rara externa (3) Pores in filtration slit diaphragms of the visceral epithelium

1. Function In 24 hours, 180 liters (48 gallons) of plasma filter out of the glomerular capillaries into Bowman's space where it is then called the filtrate. Subsequent processing of this filtrate by the tubules will result in reabsorption of 179 L, and elimination from the body of 1 L as urine.

LABORATORY GUIDE FOR URINARY SYSTEM - PART I I. EXTRARENAL COLLECTING SYSTEM Slide 72 was taken from a human kidney that underwent autolysis and is less than ideal for learning histologic features. It is useful to view this section grossly holding the slide against a white background to distinguish the lobated pattern of the human kidney. Note several papillae and minor calyces. Check to see if the layers described in lecture are present in the minor calyx.

shows proximal and distal sections of a ureter. Identify the layers, noting especially the transitional epithelium. How can you tell which one was nearest the kidney and which one was nearest the bladder? Also, do you see a serosa?

is a section of bladder with a piece of ureter included in the lumen. Review all layers of both structures. The bladder epithelium appears better preserved than that of the ureter. Note the dense irregular connective tissue of the lamina propria, and in the bladder, its vascularity. How many muscle layers are there in the muscularis? Is there a serosa present?

II. ZONES OF THE KIDNEY This is best performed on the perfused unilobate rat kidney found on Slide 74. First examine the kidney grossly and then with an inverted ocular to locate cortex, outer medulla (outer zone), and inner medulla (inner zone). This will be fun and challenging since the sections distributed through the class have many different orientations and, in

8 some cases, don't have all zones included. If you find you have been deprived of a zone, harass your immediate neighbors for their kidneys. The ideal section is shown below in cut A. Less ideal sections occur with cuts that are superficial or oriented strangely, shown in cut B.

Arcuate veins will appear as large, empty circular profiles and serve to delineate cortex from medulla which can't be distinguished grossly on the basis of texture or color. The medulla is subdivided into several zones and stripes. The outer zone of the medulla is clearly distinguishable from the inner zone, which is light and has a fine texture. The inner zone is the portion referred to grossly as the papilla. The outer zone is further subdivided into stripes. The outer stripe is very eosinophilic due to the presence of large, eosinophilic proximal tubules. The inner stripe is paler due to the absence of proximal tubules. Thus, while the outer medulla is two-toned with its two stripes, the inner medulla appears as a more homogenous area.

The medullary zones are ill-defined in the human kidney and need not be studied at this time today. Instead, use the rest of this period to identify, in rat and human, the cortex and detailed features of glomerular architecture.

III. THE RENAL CORPUSCLE Slide 74 - rat kidney, H&E Slide 75 - perfused human kidney, H&E Slide 76 - perfused human kidney, PAS Features of the renal corpuscle are easy to find in either rat or human. Distinguish the approximate inner limit of the cortex by the presence of arcuate veins and arteries. The former have extremely thin walls for their size. Can you find interlobular vessels coming off these arcuates at right angles to the surface? At low magnifications you will see the cortex to be occupied by renal corpuscles appearing as large circular profiles interspersed among the tubules. It will necessary to look at quite a few to locate all necessary features not usually present in one cross section. Be sure you can identify: 1. Bowman's space 2. Bowman's capsule - Parietal and visceral epithelium 3. Capillaries - lumens easy to distinguish, endothelial cells and walls less so 4. Vascular pole a. Afferent/efferent arterioles b. Extraglomerular mesangial cells 5. Urinary pole with proximal tubule 6. Intraglomerular mesangial cells

If a specific stain for renin were used, in which of these structures would you predict a positive reaction? What's the difference between podocytes and visceral epithelium? Or is there any?

To save yourself wear and tear in Part II, be sure you have recognized 4a and b, components of the juxtaglomerular apparatus (JGA) to be described in the second lecture. In a few lucky places, you will also be able to see another feature of the JGA, the macula densa. Look for a tubule in close apposition to this region, and if you can see cells on the glomerular side of this tubule where the nuclei are very close together forming a "dense spot", you have identified the macula densa. In your next lab session, this feature will be valuable for you to give positive identification to the distal thick limb.

9 Once you have studied renal corpuscles with H&E, repeat the procedure, using slide 106, human kidney stained with PAS, which highlights basement membranes and glycoprotein in connective tissue. This stain highlights the GBM, the mesangial matrix (which is otherwise difficult to see normally), and the Bowman's capsule.