Robert A Kyle Morie A Gertz

The word amyloid was first coined in 1838 by Schleiden, a German botanist, to describe a normal constituent of plants. Virchow [1] observed the similarity of the staining properties of the amyloid to those of starch and named it amyloid.

All forms of amyloid appear homogeneous when viewed under a light microscope and are pale pink when stained with hematoxylin-eosin. Under polarized light, amyloid stained with Congo red dye produces the characteristic apple-green birefringence. The modification of alkaline Congo red dye by Puchtler and Sweat [2] is used most often. The amorphous hyalinelike appearance of amyloid is misleading because it is a fibrous protein. On electron microscopy, amyloid deposits are composed of rigid, linear, non-branching fibrils 7.5- to 10-nm wide and of indefinite length. The fibrils aggregate into bundles. The deposits occur extracellularly and ultimately lead to damage of normal tissue.

In primary amyloidosis (AL) the fibrils consist of the variable portions of monoclonal (k) or (X) immunoglobulin light chains or, very rarely, heavy chains. In secondary amyloidosis (AA) the fibrils consist of protein A, a non-immunoglobulin. In familial amyloidosis (AF) the fibrils are composed of mutant transthyretin (prealbumin) or, rarely, fibrinogen or apolipoprotein. In senile systemic amyloidosis the fibrils consist of normal transthyretin. The amyloid fibrils associated with long-term dialysis (A ^M dialysis arthropathy) consist of ^-microglobulin.

Amyloid P component is a glycoprotein composed of 10 identical gly-cosylated polypeptide subunits, each with a molecular weight of 23,500 and arranged as two pentamers. The liver produces human serum amyloid P (SAP) component. SAP is present in healthy persons and shows 50% to 60% homology with C-reactive protein. SAP is bound to the amyloid fibrils; it is not an integral part of the fibrillar structure. It is found in all types of amyloid, including the vessel walls in patients with Alzheimer's disease. The physiologic function of SAP and its pathologic role in amyloidosis are unknown. Glycosaminoglycans are present in amyloid deposits. Their role also is unknown. Catabolism or breakdown of the fibrils is an important factor in pathogenesis; however, little is known of the process [3].

No obvious predisposing condition is associated with primary amyloidosis. Secondary amyloidosis is associated with an inflammatory process, malignancy, and many other conditions. No monoclonal protein exits in the serum or urine.

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