Project VI: Goodpasture's Syndrome

PART B:

Use LM and EM images and schematic diagrams to illustrate the structure of the Loop of Henle of the kidney. Describe in detail the function(s) of the Loop of Henle in urine production.

Loop of Henle


The Loop of Henle is formed by four different regions of the nephron;
  1. Thick Descending Limb
  2. Thin Descending Limb
  3. Thin Ascending Limb
  4. Thick Ascending Limb

Differences in the permeability, transport carriers, and channels in each segment allows for the selective movement of water and solutes into and out of the lumen of the nephron. These differences create a counter-current system which plays a critical role in the nephron's ability to concentrate urine and maintain a vertical osmolarity gradient in the medullary interstitium.

Nephron.jpg
Figure 1. The nephron is composed of several different segments. Differences in each of these segments to the permeability of water and solutes allows the nephron to produce concentrated urine.



Thick Descending Limb


The Thick Descending limb of the Loop of Henle is essentially a continuation of the Proximal Convoluted Tubule (PCT) and shares many structural and functional characteristics with it (Figure 3). As with the PCT, the Thick Descending limb is composed of simple cubodial epithelium. The apical surface of the epithelium is covered in microvilli giving it a characteristic brush border appearance (Figure 4). Functionally, the microvilli increase the surface area of the luminal surface of the epithelium, thereby increasing the Thick Descending Limb's ability to reabsorb solute from the filtrate.

TDL3.png
Figure 2. The Thick Descending Limb reabsorbs Sodium, Potassium, Chloride, Calcium, glucose, and amino acids. Due to the presence of numerous aquaporins in the epithelial lining, water passively follows the various solutes out of the lumen resulting in an iso-osmotic filtrate.
As with the PCT, the most important solute reabsorbed in the PT is the sodium cation, Na+. The electrochemical gradient of sodium favors movement into the cell and is used for the secondary active reabsorption of other solutes. For example, both glucose and amino acids are reabsorbed via co-transports with sodium. The favorable electrochemical gradient for the reabsorption of sodium is maintained by Na+/K+ ATPase pumps located on the basolateral surface of the cell. Numerous foldings along the basolateral surface of the epithelium help increase the surface area, and thus the number of Na+/K+ ATPase pumps present.

The reabsorption of Na+ from the lumen of the nephron creates an osmotic pressure difference across the Thick Descending limb favoring the movement of water out of the lumen. Aquaporin channels present on the luminal surface of the Thick Descending Limb then allow for the passive movement of water across the cell membrane and into the interstitium. Thus, the filtrate in the Thick Descending Limb is iso-osmotic with the surrounding tissue. The reabsorption of water from the lumen of the nephron then creates a favorable concentration gradient for the reabsorption of other solutes such as urea, potassium, calcium, chloride, phosphate, and bicarbonate.



Histology of the Thick Descending Limb


distal_medullary_ray_pic.png
Figure 3. Slide showing the relationship of the Medullary Ray to the Distal and Proximal Convoluted Tubules. The black bar spans across the Medullary Ray, which is representative of the Loop of Henle segments in the Renal Cortex. Immediately right and left of the Medullary Ray the Pars Convoluta can be seen.
closer_medullary_ray_pic_TDL.png
Figure 4. Slide showing the Thick Descending Limb of the Loop of Henle. Notice the microvilli brush border in the Thick Descending Limb which acts to increase the surface area of this segment to aid reabsorption and differentiates it from the Thick Ascending Limb.



Thin Descending Limb


The iso-osmotic filtrate leaving the Thick Descending Limb then enters the Thin Descending Limb (Figure 6) which is composed of simple squamous epithelium. As the Thin Limb descends into the medulla, the osmolarity of the medullary interstitium increases. Therefore water, which is able to easily cross the epithelium of the Thin Descending Limb, is reabsorbed from the lumen of the nephron to the interstitium. The amount of water reabsorbed however is small compared to the fluid in the interstitium and thus does not affect its osmotic gradient.
ThDL2.png
Figure 5. The Thin Descending limb is relatively impermeable to solutes and very permeable to water. Thus water is reabsorbed from the filtrate to the interstitium which is hyper-osmotic. Therefore the filtrate leaving the Thin Descending Limb is hyper-osmotic


While water can cross the epithelium on the Thin Descending Limb, the epithelium is relatively impermeable to solute. Therefore the filtrate in the Thin Descending Limb becomes hyper-tonic due to the reabsorption of water. The length of the Thin Descending Limb various among nephrons and therefore so to does the ability to concentrate urine. Cortical nephrons located in the cortex near the capsule of the kidney have a shorter Thin Descending Limb than do Juxtamedullary nephrons located near the medulla. Thus, Juxtamedullary nephrons are able to concentrate the urine more than Cortical nephrons.


Thin Ascending Limb


Next, the filtrate enters the Thin Ascending Limb (Figure 6). While histologically very similar to the Thin Descending Limb (i.e. both segments are composed of simple squamous epithelium), the Thin Ascending Limb is impermeable to water. Therefore as the Thin Limb ascends back up towards the cortex, water is unable to move from the hyper-tonic filtrate to the medullary interstitium.


Histology of the Thin Descending and Thin Ascending Limbs


thin_limb.png
Figure 6. Slide showing the thin segment of the Loop of Henle. This is determined by the simple squamous epithelial lining of the thin segments of the Loop of Henle versus the cuboidal epithelium of the Thick Descending and Ascending Limbs of the Loop of Henle.




Thick Ascending Limb


Finally, the hyper-tonic filtrate enters the Thick Ascending Limb (TAL). Like the Thick Descending Limb, the TAL consists of simple cubodial epithelium. However the cells of the TAL lack microvilli allowing it to be histologically distinguished from the cells of the Thick Descending Limb (Figure 8).

TAL2.png
Figure 7. The Thick Ascending Limb removes Sodium, Potassium, Chloride, and Calcium from the lumen of the nephron by both active and passive reabsorption. The epithelium on the Thick Ascending Limb however are impermeable to water, resulting in an hypo-osmotic filtrate inside the lumen of the nephron.


In the TAL, a Na+/K+/Cl- co-transporter uses the favorable electrochemical gradient of sodium for the reabsorption of solute from the lumen of the nephron. As before, this favorable electrochemical gradient is maintained by Na+/K+ ATPase pumps located on the basolateral surface of the epithelium. Furthermore, the Na+/K+/Cl- co-transporter establishes a positive trans-epithelial potential which favors the para-cellular reabsorption of both K+ and Ca++.

The Thick Ascending Limb however is impermeable to water. Thus the net removal of solute from the TAL causes the filtrate to become hypo-tonic. Furthermore, the net removal of solute from the TAL helps to establish the vertical concentration gradient in the medullary interstitium. This last point is discussed in more detail in the section on the Counter Current Multiplier.


Histology of the Thick Ascending Limb


closer_medullary_ray_pic.png
Figure 8. Slide showing the Thick Ascending Limb of the Loop of Henle. The Thick Ascending Limb differs from the Thin Descending Limb by the absence of a microvilli brush border.


Summary


To summarize,
  • In the Thick Descending Limb, both water and solute are reabsorbed from the filtrate resulting in an iso-osmotic filtrate.
  • In the Thin Descending Limb, water is reabsorbed from the filtrate resulting in an hyper-tonic filtrate.
  • In the Thin Ascending Limb, the epithelium of the nephron is impermeable to water preventing the back flow of water to the interstitium.
  • In the Thick Ascending Limb, solute is reabsorbed from the filtrate resulting in a hypo-tonic filtrate.


Counter Current Multiplier


The Loop of Henle plays an important role in establishing the vertical osmotic gradient in the medullary interstitium through the counter current multiplier. The transport mechanisms in the TAL are able to establish a 200mOsM/L difference between the lumen of the nephron and the interstitium. Due to the flow of filtrate in the Loop of Henle, this horizontal gradient of 200mOsM/L creates a 400mOsM/L vertical osmotic gradient. Hence the name counter current "multiplier".

counter.png
Figure 9. Differences in the permeability of the descending and ascending portions of the Loop of Henle create the Counter Current Multiplier which helps establish a vertical osmotic gradient in the interstitium


To understand how this works, imagine a volume of filtrate flowing through the Loop of Henle. As the filtrate enters the Thin Descending Limb it is iso-osmotic and therefore water is reabsorbed into the hyper-osmotic interstitium. The amount of water leaving the lumen is relatively small and therefore does not affect the osmolarity of the interstitium. As the volume of filtrate continues down the Loop of Henle water is continuously reabsorbed due to the increasing osmolarity of the interstitium. Therefore at the end of the Thin Descending Limb the filtrate is hyper-tonic.

In the ascending portion of the Loop of Henle, solute is then reabsorbed from the filtrate to the interstitium. Thereby reducing the osmolarity of the filtrate and increasing the osmolarity of the interstitium. This process continues throughout the ascending portion of the Loop of Henle. Therefore as a volume of filtrate ascends up the Loop of Henle it's osmolarity is constantly decreasing. Despite the changing osmolarity of the filtrate, the transport mechanisms in the TAL continue to maintain a constant 200mOsM/L horizontal gradient thereby creating a vertical gradient in the interstitium.

CT2.png
Figure 10. The counter-current multiplier is only responsible for approximately half of the osmotic gradient in the medullary interstitium. The remainder of the osmotic gradient is created by the recycling of urea from the collecting tubule to the interstitium and then back to the Loop of Henle.


In addition to the counter-current multiplier, the osmolarity of the interstitium is maintained by the net recycling of urea from the Collecting Tubule (Figure 11) to the Loop of Henle. The recycling of urea creates an additional vertical gradient in the interstitium that accounts for approximately half of the overall vertical osmotic gradient in the interstitium.

closer_collecting_tubule_pic.png
Figure 11. Slide showing the Collecting Tubules of multiple Nephrons. In histological slides, the Collecting Tubules have a lighter color, while the Thick Descending and Ascending Limbs are darker in color. In addition, the Collecting Tubules are significantly larger than the Loop of Henle segments.



Counter Current Exchange


Normally if a tissue becomes hyper-tonic, the extra solute diffuses into and is carried away by the surrounding vasculature. To avoid this, the osmolarity of the interstitium is maintained by a counter current exchange system formed by the vasa recta. The vasa recta are capillaries which form a loop extending from the cortex into the medulla and returning back to the cortex. Since the vasa recta is permeable to both water and solute, the water and solute lost/gained as the vasa recta descends into the medulla is equal to the water and solute gained/lost as the vasa recta ascends back into the cortex.

vasa.png
Figure 12. The path of the Vasa Recta creates a loop through the cortex and medulla of the kidney. Since the Vasa Recta is permeable to both water and solute, the water/solute lost/gained by the blood as it descends into the medulla is equal to the solute/water gained/lost as it ascends back up to the cortex. Thus persevering the vertical osmotic gradient in the medulla.


To be more specific, as blood flows down the vasa recta into the medulla, the osmolairty of the surrounding interstitium increases. Therefore
  1. Water moves from the vasa recta to the interstitium due to the osmotic pressure difference.
  2. Solute moves from the interstitium to the vasa recta due to the concentration difference.
  3. The blood becomes hyper-tonic.
As the hyper-tonic blood flows up the vasa recta into the cortex, the osmolarity of the interstitium decreases. Therefore
  1. Water moves from the interstitium to the hyper-tonic blood in the vasa recta due to the osmotic pressure difference.
  2. Solute moves from hyper-tonic blood in the vasa recta to the interstitium due to the concentration difference.
  3. The blood returns to its normal osmotic level.
The net result is that the osmolarity gradient of the interstitium is maintained.




PART A:

Describe the molecular mechanism underlying Goodpasture Syndrome. Find appropriate LM or EM images and/or schematic diagrams to show the components involved in this disease, and label them. Describe the presentation of the disease (i.e. the symptoms that the patient displays), its progression, and its treatment (if any). Include a representative LM that shows the pathology (the University of Iowa virtual microscope and our Pathology slide box may be useful here).



Goodpasture Syndrome (GS) is an example of an antibody-mediated disease via Type II hypersensitivity. The target antigen is noncollagenous protein in the basement membranes of kidney glomeruli and lung alveoli. Antibodies target the α3 chain of collagen type IV. Such antibodies can be detected in serum of roughly 90% of persons with Goodpasture syndrome. Inflammation is mediated by complement and Fc receptor. Due to such reactions in the kidney and lungs, the clinicopathologic manifestations can include nephritis and lung hemorrhage.

This condition begins with the development of anti-GBM antibodies which causes structural changes within the glomeruli in the kidney (Fig. 13,14,15) and within the alveoli in the lungs (Fig. 16,17).


Symptoms are reflective of the disease's progression and may include either respiratory or renal abnormalities, as well as simultaneous respiratory and renal abnormalities. Early detection of the disease highly correlates with patient survival and renal survival.

Respiratory and renal abnormalities can be identified by a complete patient history and physical examination. This syndrome is often triggered by inhalation of chemical irritants or a viral respiratory infection. Patients may present with symptoms such as nausea and vomiting, skin pallor, fatigue, chest pain, cough, hemoptysis, dyspnea, hematuria, dark colored urine, foamy urine, and decreased urine output. Physical examination may reveal hypertension, edema, and abnormal lung sounds (crackling).

Other necessary clinical tests are required for diagnosis. Chest radiography can identify alveolar disruption and pulmonary hemorrhage. Measurement of carbon monoxide transfer factor during a pulmonary function test may also be used as further evidence for GS. Urinalysis can identify blood and protein in the urine when renal function is compromised. Blood analysis can evaluate kidney function based on plasma creatinine concentration. Blood tests may also reveal the presence of circulating anti-GBM antibodies. Elevation of anti-GBM antibodies is the key factor in diagnosing GS. Lastly, a renal biopsy will be performed to assess the extent of diseased glomeruli, called crescentric glomerulonephritis.

GS is often overlooked in its earliest stages because of the vagueness of its symptoms. Combined with its rapid progression, it is often not caught until the more severe symptoms appear. For example, respiratory abnormalities may begin as a dry cough with short periods of dyspnea and may progress until the individual becomes hypoxic or has pulmonary hemorrhaging. However, GS does not usually result in permanent lung damage. The onset of renal disease begins with glomerular nephritis, which results in loss of filtration selectivity. Proteinuria and hematuria ensues from the damaged glomeruli. These symptoms are early signs of acute renal failure and its progression includes high blood pressure and severe edema.

If left untreated, patients will not recover renal function and have high mortality rates. Therefore, early diagnosis is very important for patient survival and renal survival. GS is treated with a regimen of immunosuppressive drugs and plasmapheresis. Prednisolone and cyclophosphamide limit immune response and prevent the production of autoimmune antibodies that target one’s own tissue. Corticosteroids are also given to control pulmonary hemorrhaging and reduce immune response. Plasmapheresis is required to eliminate and decrease the concentration of circulating anti-GBM antibodies from the blood. If the disease is diagnosed after renal function has been compromised, kidney dialysis must be performed to remove waste products from the blood. Treatment of GS is considerably less effective in patients with more severe renal failure than those who began treatment at earlier stages.

Z_Renal_Corpuscle-_Slide_4_(schematic).png
Figure 13. Schematic of the renal corpuscle, the site at which Bowman's capsule and the glomerulus meet. Note: Urinary space = Bowman's space. (University of Maryland)



Renal_Corpuscle-_Slide_1_(LM).png
Figure 14. LM image of the renal corpuscle. (Southern IL University Med School)



Z_Glomerular_capillaries-_Slide_3_(EM).png
Figure 15. EM image displaying the filtration membrane (comprised of the endothelial and glomerular (podocytes) basement membranes) which separates the capillary lumen from Bowman's space in the renal corpuscle. (Southern IL University Med School)



Z_Alveoli-_Slide_9_(LM).png
Figure 16. LM of alveoli (A). (RFUMS Odd-54B)
Z_Alveoli-_Slide_7_(EM).png
Figure 17. EM displaying alveoli (Al) in the lung. (University of Kansas)




Charytan, David B. “Goodpasture’s Syndrome.” MedlinePlus. 2006. American Accreditation HealthCare Commission. 7 Dec. 2008 <http://www.nlm.nih.gov/medlineplus/ency/article/000142.htm>.

Costanzo, Linda S., "Physiology, Board Review Series" Lippincott Williams & Wilkins, Fourth Edition, 2007

Eaton, Douglas C. and Pooler, John P. "Vander's Renal Physiology" Lange Medical Books, Sixth Edition, 2004

Levy, Jeremy B., et. al. “Long-Term Outcome of Anti-Glomerular Basement Membrane Antibody Disease Treated with Plasma Exchange and Immunosupression.” Annals of Internal Medicine. 134 (2001): 1033-1042.