Renal Anatomy-Histology Correlate

1. RenalAnatomy-Histology Correlate By: Michael Lu, Class of ‘07

2. NOTE: • Near midline, the vena cava, abdominal aorta, and the esophagus entering through the diaphragm. The aorta is slightly left of midline, while the vena cava is on the right side. • The kidneys are situated anterior to the diaphragm resting in the paravertebral gutters formed by the muscles of the posterior abdominal wall. • The right kidney is lower than the left kidney because it is pushed down by the liver. • The adrenal or suprarenal glands resting above the kidneys. The glands are supplied by superior, middle, and inferior suprarenal arteries. • The left renal vein, as it crosses over the aorta and under the superior mesenteric artery, is longer than the right renal vein. • The right renal artery is longer as it passes posterior to the vena cava.

3. NOTE: • The diaphragm separates the lungs from the abdominal cavity. • The kidneys located in the retroperitoneum, embedded in considerable amounts of fat. • The renal fascia completely surrounding both the kidney and adrenal gland. • The lower half of the kidney resting on the paravertebral gutters formed by the muscles of the posterior abdominal wall on either side of the vertebrae. • Pararenal (around renal fascia) versus perirenal (inside renal fascia) fat. The perirenal fat is continuous with the fat found in the kidney hilum and renal sinus.

4. NOTE: • The outer renal cortex is paler in color and extends down as renal columns between separate renal pyramids, which make up the renal medulla. • Each renal pyramid together with the cortex overlying it constitutes a renal lobe. • The base of the renal pyramid is closer towards the outer fibrous capsule of the kidney, while the apex of the pyramid empties into the renal papilla. • The renal papilla empty into minor calices, which converge to form major calices. The space that is occupied by these renal structures, vessels, nerves and a variable amount of fat is known as the renal sinus. • The major calices empty into the renal pelvis, which then leave the hilum of the kidney as ureters to the bladder, where the urine is expelled. • Medullary rays within the cortex will be covered later.

5. As the renal artery enters the hilum of the kidney, it branches into segmental or lobar arteries. These give rise to interlobar arteries. • The interlobar arteries then take 90 degree turns at the junction of the renal cortex and medulla and are called arcuate arteries. • The arcuate arteries then take another bend and travel up the cortex as interlobular arteries (cortical radiate) in between the renal pyramids up the renal columns. • Afferent arterioles branch off from the interlobular arteries and drain into individual renal corpuscles, where filtration takes place. The are shown in very low power here and will be discussed in more detail in the following slides.

6. As a general rule, the renal cortex is more convoluted in appearance, while the medulla consists of primarily straight tubules. • Vessels that are running in the plane of the cortico-medullary junction are arcuate vessels, either in cross or longitudinal section. On the top left panel, the arcuate vessels are highlighted in the box. The arrows point to interlobular vessels within the renal cortex. • The interlobular artery branches off from the arcuate artery and gives off intralobular arteries (bottom panels). • The intralobular artery gives rise to afferent arterioles, feeding into various glomeruli (plural for glomerulus) or renal corpuscles.

7. The glomerulus marks the starting point of the basic unit of the kidney – the nephron. • Blood flows from an afferent arteriole into the glomerulus, where it is filtered before leaving via an efferent arteriole. • The filtrate is captured in the Bowman’s capsule, which successively enters the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and finally the collecting tubule and duct. • The loop of Henle consists of both descending and ascending limbs extending into the medulla. As shown in the diagram on the right, different portions of the loop have different permeabilities, thus allowing the loop of Henle to selectively remove salts and water and to create an osmolarity gradient. This is known as a counter-current multiplier system. • The arcuate arteries also give off some vasa rectae that loop down into the medulla with the loops of Henle. Because these capillaries pass through this region of high osmolarity, there is an exchange of blood fluid/water. Together, they are known as the counter-current exchange system. • The steroid hormone, aldosterone, from the adrenal cortex regulates salt retention in the nephron. The peptide hormone, antidiuretic hormone or ADH, regulates water permeability of the collecting duct and influences the final volume and concentration of urine.

8. In the top panel, the border between the renal cortex and medulla, or cortico-medullary junction, is indicated by the dashed line. As mentioned before, the cortex contains tubules cut in both cross and longitudinal sections, while the medulla consists of tubules all primarily in the same direction. • Within the cortex we see the pars convoluta, which are the proximal convoluted tubules of the nephrons. • The pars radiata, also known as medullary rays, run longitudinally within the cortex. They appear very similar to the tubules within the medulla, and are essentially the loops of Henle and collecting tubules of the nephrons that are positioned higher within the cortex. They allow the renal corpuscles, or glomeruli (indicated by the blue arrows in the lower panel) to be placed at different heights within the cortex and to be more closely packed together. • An artery that supplies the corpuscles is labeled. It also serves as the boundary for the lobule. The corpuscles will be examined more closely in the following slides.

9. The renal corpuscle is shown here. The outer aspect consists of a thin connective tissue capsule called Bowman’s capsule. • Bowman’s capsule is composed of a parietal layer, which are simple squamous epithelial cells, and a visceral layer, or podocytes. • The glomerulus “proper” is a convoluted tuft of capillaries containing regular endothelial cells and the podocytes (discussed in more detail in the next slide). • In the top panel, the vascular pole of the corpuscle is indicated by the arteriole. The urinary pole is indicated by the beginning of the proximal tubule (p.t. begin). • The proximal convoluted tubules are identified primarily by their microvilli brush border, giving the lumen a “fuzzy” appearance. The cuboidal cells are large, prominent, and darker staining. There are more proximal tubules seen simply because they are longer and more convoluted. • Distal tubules generally have a wider lumen and lack a brush border. Near the corpuscle, the distal tubule has a thicker group of cells known as the macula densa. • The mesangial cells maintain the basement membrane and regulate blood flow through the glomerulus.

10. As mentioned earlier, blood filtration occurs within the renal corpuscle. • The renal blood filtration barrier consists of three parts: • capillary endothelium – fenestrated • fused glomerular basement membrane • podocytes – the foot processes known as pedicels wrap around the capillary network in the glomerulus and form filtration slits • The filtration barrier allows the free passage of salts, sugars, and many other small molecules, but it does not allow the passage of proteins or blood cells. • The composition of the fused basement membrane is very crucial. In many renal diseases, it is the common source of defects. • The glomerular filtrate is collected within the Bowman’s space and drained from the renal corpuscle at the urinary pole, entering the proximal convoluted tubule. • The loop of Henle then processes the filtrate within the medulla.

11. The medulla of the kidney is characterized by numerous longitudinally oriented tubules composed of descending and ascending limbs of loops of Henle, collecting ducts, and vasa rectae. • The descending limb of the loop of Henle is more similar in appearance to the proximal tubules in the cortex. It contains a “fuzzy border”. • The ascending limb looks more like the distal convoluted tubules with low cuboidal epithelium. • The collecting duct is composed of a simple cuboidal epithelium. The nuclei are very prominent, and there is a clear demarcation between cells, as indicated by the small arrows. • The vasa rectae are discussed next.

12. The vasa rectae are small capillaries that branch off of the efferent arterioles within the kidney. They are longitudinally oriented towards the medulla and lie adjacent to the loops of Henle. • In a lower magnification, the vasa rectae appear as reddish or brownish stripes in the medulla, as shown in the bottom panel. In the top panel, the specimen has been injected with a dye to accentuate the blood vessels. • The afferent arterioles lead to renal corpuscles, where the blood is filtered, and leaves via efferent arterioles. Many components of the filtrate are valuable to the body and need to be reabsorbed back into the bloodstream. The close proximity of the vasa rectae with the loops of Henle, within the high osmolarity environment of the medulla, allows the counter-current exchange system to reabsorb compounds from the glomerular filtrate. • The vasa rectae return blood to interlobular or arcuate veins, which will lead to the renal veins and back into the system circulation.

13. At the end of the distal convoluted tubule, there is a thickening in the epithelium that signifies the macula densa. It acts as a sensor of the composition of the filtrate after it has been processed and provides feedback regulation. • The macula densa is part of the juxtaglomerular apparatus (JGA) that plays an important role in the control and regulation functions of the kidney. It can be found between the afferent and efferent arterioles near the glomerulus. • The kidney, or more specifically the JGA, is crucial in the regulation of blood volume, pressure, and osmolarity.

15. As a review: • Note the abdominal aorta to the left of midline and the vena cava to the right. The aorta is shorter. • The kidneys are situated anterior to the diaphragm resting in the paravertebral gutters of the posterior abdominal wall. • The adrenal (suprarenal glands) resting above the kidneys. The right kidney is lower than the left kidney because it is pushed down by the liver. • The left renal vein, as it crosses over the aorta and under the superior mesenteric artery, is longer than the right renal vein. • The right renal artery is longer as it passes posterior to the vena cava. • - The collecting ducts, with the processed filtrate, continue to merge and form larger ducts as they approach the apex of the renal pyramid, or renal papilla. The largest ducts empty into the calices. • - The papilla is covered with transitional epithelium. It empties into the ureters. • - The ureters exit from the hilum of the kidneys and travel posteriorly to the pelvis. Note how they travel behind the gonadal vessels but in front of the iliac vessels.

16. The ureter is lined with transitional epithelium, which sits on a lamina propria that blends with the underlying muscular layer. • There is no submucosa.The adventitial layer is distinct. • The ureter has a rather disorganized smooth muscle layer. • Both a relaxed (top panels) and distended (bottom panels) ureter are shown here at low and high magnifications. It exhibits the thinning of the epithelium and widening of the ureter lumen, emphasizing the distensibility of the transitional epithelium.

17. The bladder serves to store urine. Like the ureters, it is also lined with transitional epithelium, which is shown at higher magnification on the bottom left panel. • The transitional epithelium also maintains a barrier between the hyperosmotic urine in the lumen and the capillary network in the lamina propria. • The muscular wall of the bladder, which consists of three layers of smooth muscle, provides the force for urine expulsion during urination. • The three smooth muscle layers are normally difficult to distinguish (top left panel). In the bottom right panel, the Masson trichrome stain differentiates between the inner longitudinal, outer circular, and outer longitudinal layers.