Amyloidosis

Introduction

Amyloid infiltration occurs in all species - amyloid is an inert substance that becomes deposited under the endothelium and basement membranes of a variety of tissues, notably the renal glomeruli, Islets of Langerhans in the pancreas and the liver (between the sinusoidal reticulum and the hepatic cords).

Causes

May be a primary condition or appear secondary to some chronic tissue destructive process such as:

• an infectious process elsewhere in the body
• sustained antigenic stimulation, eg repeated injections with an antigenic substance or production of excessive antibody by leukocytes

Gross Pathology

Affected organs will apear:

• pale (greyish waxy appearance)
• enlarged with rounded edges
• firm
• very prone to rupture

Microscopically

• deposition of amyloid in the space of Disse (or perisinusoidal space) which is in the liver between the hepatocytes and a sinusoid.
• shows 'apple-green' fluorescence under polarised light after staining with Congo Red

Arterial Dissemination

Amyloid is an eosinophilic, homogenous, hyaline material. Due to its beta-pleated-sheet structure it is almost insoluble. Amyloid may be present in one of two forms:

• AA: Serum amyloid A, alpha-2 globulin.
• AL: Derived from immunoglobulin light chains.

Disease may be truly idiopathic (dogs and cats) or may be secondary to another disease process, often chronic inflammation or neoplasia. Chronic antigenic stimulation induces the overproduction of AA protein which may become deposited throughout the body.

Deposits can be found in:

• Renal vessels and glomeruli.
• Splenic white pulp.
• Space of Disse.
• Coronary arteries.
• Meningeal arteries.

Affected organs are non-functional and appear waxy and pale.

• Amyloid is an insoluble protein that can accumulate in the kidney and compress the glomerulus, interfering with its normal function.
• Hypoproteinaemia and nephrotic syndrome can result due to substantial protein loss in the urine.
• Causes can be idiopathic or associated with underlying chronic inflammatory conditions.

Gross pathology

• Kidneys are firm, enlarged, and pale. Affected glomeruli may be seen as yellowish spots in the cortex.
• Amyloid can be visualised by treating the freshly cut surface of the kidneys with iodine.

Histopathology

• Amyloid stains pink with congo red.
• Presence of a pink, homogenous material replacing the epithelium and endothelium of the glomerulus.

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  Amyloidosis Stained with Congo Red
  Susan Rhind, University of Edinburgh

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  Amyloidosis Histology
  Susan Rhind University of Edinburgh

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