Urinary System

Samantha Blum

The Urinary System

• The urinary system functions primarily in osmoregulation and excretion of toxic metabolic waste products. In this laboratory, we will examine the components of the urinary system: the kidneys, ureters, bladder, and urethra.

Kidney:Overview & Anatomy

• The two kidneys lie behind the peritoneum on either side of the upper lumbar vertebrae, embedded in fat in the paravertebral gutters of the posterior abdominal wall (dissection). The left kidney usually lies at a higher level than the right.

• Covered by a thin capsule, the kidney comprises an outer cortex and an inner medulla (schematic; dissection). The medulla contains numerous pyramids; the apex of each pyramid, called a renal papilla, projects into the renal sinus. Urine discharged from the papillae is collected by about ten trumpet-shaped chambers, the minor calyces, which unite to form two or three major calyces. These then fuse into the single, funnel-shaped renal pelvis, which is continuous with the ureter.

Kidney:Cortex

• Identify the cortex below the kidney capsule (4X).• At higher magnification, identify the regions of the cortex. Note that the renal corpuscles (4X) are readily

visible.• Identify medullary rays (4X, 4X, schematic) alternating with regions containing renal corpuscles and their

convoluted tubules (pars convoluta). Note that medullary rays are actually in the cortex. On the trichrome-stained slide (O-031), the middle section is a cross-section of the kidney cortex that illustrates this relationship very well.

Kidney:Medulla

• Identify medullary pyramids, best seen in O-062B (1X).• Identify the outer medulla and the inner medulla, best seen in slide SCPM070 (1X). The "outer stripe" and

"inner stripe" together comprise the outer medulla. What structural features make the inner and outer stripes look different from each other (hint)?

• Identify papillae (1X, 4X).

Kidney:Renal Sinus

• Identify minor calyces, best seen in O-062A and SCPM -070 (2X, 10X). Make sure you understand the relationship of minor calyces to the major calyces and renal pelvis.

• Identify the renal pelvis.

Kidney:Uriniferous Tubules

• Each uriniferous tubule is comprised of a nephron and a collecting tubule (schematic). Although of different embryological origins, these two tubular structures unite during development to form an anatomical and functional unit. The blind start of the uriniferous tubule, Bowman's capsule, is intimately associated with a vascular structure, the glomerulus. Together, these structures form the renal corpuscle. At this site, an ultrafiltrate of blood enters the lumen of the nephron, and the process of urine formation begins. The nephron has several regions, each with its own distinct function in the production of urine (table).

Renal Corpuscle• Identify a glomerulus and the

surrounding Bowman's capsule (20X, 40X).

• Look for a vascular pole (40X, 40X) and a urinary pole (40X, 50X).

• Identify an afferent arteriole (10X, 50X); you can be sure of its identity if it branches from an intralobular artery. You can try to find an efferent arteriole, but this is only possible if you find a lucky section that contains both types of vessel.

• The visceral and parietal layers of Bowman's capsule form during development, when a knot of capillaries invaginates into the tip of a primitive renal tubule, much like a finger pushing into an inflated balloon (schematic).

Vascular Pole of Renal Corpuscle

Kidney:Proximal Convoluted Tubules

• Proximal convoluted tubules are located around the renal corpuscles in the pars convoluta, but are not present in the medullary rays.

• Look for a proximal convoluted tubule emerging from the urinary pole (40X) of Bowman's capsule.

• Note that proximal convoluted tubules (20X, 50X) have simple, low columnar epithelium with a prominent apical brush border, best seen in O-030 (50X, 100X, 100X). Typically, the brush border is poorly preserved in most LM slide preparations; only remnants of it are observed in the lumen. You can identify proximal tubules by their brush border; distal tubules and collecting tubules don't have a brush border.

Kidney:Proximal Convoluted Tubules

• PCT cells have basal striations, too small to see at 40x; these represent infoldings of the basal surface membrane, into which are packed large numbers of mitochondria (arrows, EM). These infoldings increase the basal surface area of the tubule cells, much like the microvilli do on their apical surfaces. Why do these cells need such a large basal surface area? – Na/K ATPase present on the basal surface to

pump Na out into the blood (drives reabsorption of both Na and water)

– Water enters/exits the PCT at both the apical and basolateral surfaces

• The majority of tubules in regions surrounding the renal corpuscles are proximal convoluted tubules (20X, 20X). Why is this the case?– PCTs are the longest segments of the

nephron in the cortex, so there are more PCT profiles in the cortical regions containing renal corpuscles

Proximal Tubule (P) vs. Distal Tubule (D)

D

D

PP

Kidney:Loop of Henle

• Thick descending limbs of the loop of Henle. These are located in medullary rays (20X, 50X), and continue into the medulla. They are histologically similar to proximal convoluted tubules (although they are straight, not convoluted). They are also called proximal straight tubules.

Contains both thick descending and thick ascending limbs

Kidney:Loop of Henle

• Thin limbs of the loop of Henle. These loops connect the descending thick and ascending thick limbs, and are only found in the medulla. In juxtamedullary nephrons, the thin segment forms the loop of Henle, while in cortical nephrons it forms only part of the descending limb of the loop.

• Examine several regions of medulla for thin limbs, which are tubes made of simple squamous epithelium. In cross-section, the thin limbs (50X, 100X) look like empty capillaries.

• In longitudinal views, you can appreciate why the capillaries of the medulla are called "straight vessels" (vasa recta) (100X, 100X). These capillaries often contain erythrocytes, so you can distinguish them easily from the thin limbs, which are always empty.

Kidney:Loop of Henle

• Thick ascending limbs of the Loop of Henle. Also called distal straight tubules, these structures (20X, 50X, 50X) can be observed in medullary rays and the medulla.

• Note that, in cross-section, they look very similar to distal convoluted tubules (although they are straight, not convoluted).

Kidney:Distal Convoluted Tubules

• Distal convoluted tubules. These structures can be observed in the vicinity of renal corpuscles and proximal convoluted tubules.

• DCTs are considerably shorter than proximal convoluted tubules, so they are less frequent in a cross section of the cortex (20X).

• Note that DCTs have a simple cuboidal epithelium with no brush border, and an empty lumen (40X, 50X).

• Like PCT cells, DCT cells have basal infoldings with many mitochondria (arrows, EM).

Kidney:Collecting Tubules and Ducts

• This portion of the uriniferous tubule is primarily excretory in function, although some modification of urine tonicity does occur here. To identify collecting tubules, you may find it easiest to begin near the tip of the medullary papilla of slide O-062A, then follow the tubules up towards the cortex (schematic, 1X, A → B → C → D).

• Papillary ducts (20X, 40X). These are the large collecting ducts of the papillary region, and drain into the calyceal space (2X, 4X). At this level, the ductal epithelium is tall columnar.

Kidney:Collecting Tubules and Ducts

• Moving up towards the cortex, you should first encounter thin limbs as well as capillaries (vasa recta) (40X).

• Slightly higher, thick limbs appear (ascending and descending), while the collecting ducts are smaller and their epithelium has become less columnar (40X).

• Near the medulla/cortex border, the collecting ducts are almost the same size as the thick limbs, and their epithelium is cuboidal; the two types of tubule are therefore difficult to distinguish (40X). From this point on (i.e. in medullary rays), you will not be expected to distinguish collecting ducts and tubules from thick limbs.

Kidney:Juxtaglomerular Apparatus

• Macula densa. • Look near glomerular arterioles (at the

vascular pole) to identify this structure (50X, 50X, 100X), which is a specialization of the distal convoluted tubule.

• What does the term "macula densa" mean?

• Juxtaglomerular cells. • These cells are located in the afferent and

efferent arteriole walls next to a macula densa; they are modified smooth muscle cells with an epithelioid appearance (100X)

• JG cells contain renin granules, although these cannot be distinguished at 40X on a virtual slide.

• Lacis cells. Also called extraglomerular mesangial cells, these are mesangial cells that lie outside the renal corpuscle, and that form part of the JG apparatus (schematic). They cannot be reliably identified in routine H&E preparations.

Macula Densa Cells

Renal Blood Supply

• Interlobar artery (overview schematic, schematic)

• Arcuate artery. (4X, 10X, 20X)

• Interlobular artery. (schematic, 10X)

• Afferent arteriole. (10X, 50X)

Interlobular Artery

Renal Blood Supply

• Glomerulus. (4X) • Efferent arteriole. (

50X) • Peritubular capillary

plexus. (40X, 40X)

Renal Blood Supply

• Vasa recta. • Interlobular vein. (2X

, 4X, 10X) • Arcuate vein. (2X, 4X,

10X, 20X) • Interlobar vein. (2X)

Interlobular Vein

Renal Pelvis• The renal pelvis is an expanded region of ureter that forms a cap over the renal papilla. Hilum. Identify, where possible: • Renal papilla.

– Identify Ducts of Bellini (4X, 10X) opening into the pelvicalyceal space.• Minor calyx (4X, 10X) . Note the transitional epithelium lining the calyx.

Renal UreterUreter (4X). • Mucosa.

– Identify the characteristic transitional epithelium (50X).– Note the stellate (star-shaped) lumen (4X).– Identify lymphoid structures. (10X, 100X) PLASMA CELLS

• Muscularis.– Identify smooth muscle layers (4X, 10X): inner longitudinal, outer circular (and outermost oblique or

longitudinal in lower ureter).• Adventitia.

– This loose C.T. layer blends with the C.T. of neighbouring organs (10X).

Urinary Bladder• The urinary bladder is structurally similar to the ureter. Compare the relaxed and distended sections.

Mucosa. (4X, 10X) • Identify the transitional epithelium that forms the lining (40X).• Muscularis. The greatest difference between ureter and bladder is found here. • Note the significantly increased thickness of the smooth muscle tunic (4X).• Identify autonomic ganglia (40X) embedded within the muscular wall. What is their function?• Adventitia/Serosa. • Note that the exposed regions of the bladder are covered by a serosa, while those regions that are

embedded within other bodily tissues are surrounded by a loose C.T. adventitia (20X).• What are the two components of a serosa?

Histopathology:Amyloidosis

• Amyloid is the general name for proteinaceous material that accumulates as abnormal deposits within tissue spaces. Several different types of amyloid exist, but all are aggregates of polypeptides (protein fragments) that have a beta-pleated sheet structure. In brain, amyloid accumulation accompanies Alzheimer's disease; however, almost any organ can be affected.

In this kidney specimen (1X), the medulla contains aggregates of amorphous material surrounding the ducts and tubules in this region (5X, 20X). The cortex appears relatively normal. Eventually, the deposits of amyloid become so large that they obliterate the normal tissue structure.

Amyloid Deposits

Histopathology:Renal Infarct

• An infarct is an area of ischemic necrosis (tissue damage caused by lack of oxygen) caused by blockage of the arterial supply or the venous drainage in a tissue. In this case, blockage of an interlobar artery (schematic) cut off blood flow to several interlobular arteries, resulting in necrosis of several kidney lobules. Ischemic areas (1X) show severe structural damage (10X), while nearby areas appear normal (10X).

Histopathology:Polycystic Kidney Disease

• Polycystic kidney disease (specimen) is a relatively common (1/800 births) congenital, autosomal dominant disease. Fluid-filled cysts develop from the PCT, DCT, and CT; these cysts disrupt the organ's structure and function, leading to organ failure.