Buster, an 11 year old neutered male German Shepherd, has been brought to your clinic for his last visit. You've been treating Buster for chronic renal failure for the past six months and the time has come when he simply can't fight it off any longer. You have discussed his condition extensively with his owners and they have decided that humane euthanasia is the best treatment for him now. They have also decided to have Buster cremated but they would like for you to perform a necropsy in order to learn more about Buster's disease so you can treat other dogs in the future.
While a necropsy may seem like the medical equivalent of closing the barn door after the horses have run away, it is actually a very valuable and powerful tool for understanding disease processes and how to combat them. Part of the concept of the practice of medicine is to learn from the past to deal with the present and to prepare for the future—necropsy provides us with a great deal of knowledge that can be used to help our future patients.
This is a gross specimen of the kidney, stained with iodine. Iodine is strongly bound by amyloid. You can see the deposits in this specimen quite easily as dark areas, and the fibrosis in the cortex is also visible as whitish regions. The glomeruli have been completely obliterated. As this happens Buster's renal failure gets worse.
A low-magnification image of the renal cortex is shown below: notice the completely filled glomeruli and the deposition of amyloid in the renal tubules as well.
As deposition of amyloid occurs, pressure necrosis kills off the cells of the glomerular capillaries and their overlying podocytes. The loss of the podocytes means loss of the filtration barrier, hence the proteinuria that is a hallmark of renal disease. As the tubules fill with amyloid (above left) they lose their ability to recover not only proteins but glucose and sodium. The loss of distal tubules and their ability to translocate sodium to the interstitium means loss of the normal concentration gradient, and hence the inability to concentrate urine at all. Non-functional kidneys mean accumulation of toxic metabolites in the blood and eventually, death.
How can you tell the difference between amyloid and hyaline in this specimen? The two arise by different processes, and amyloid is never intracellular, it's a deposit that takes place in the intercellular spaces. Another clue is the positive reaction to iodine, which doesn't happen with hyaline. A definitive test is the use of polarized light. Amyloid is birefringent; that is, it passes light in two different directions. When you illuminate it with a polarizing microscope, it has an entirely different appearance from hyaline. At left is such a sample, the amyloid visible as brightly illuminated areas
Kidneys are often the targets of a secondary amyloidosis: that is, the amyloidosis occurs secondary to another disorder, commonly a chronic nephritis or glomerulitis. Amyloid deposition can arise in the kidney due to any number of factors and a mild idiopathic amyloidosis is not uncommon in older animals.
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