Amyloidosis: An Overview
 Amyloidosis is an abnormal condition in which protein substances are deposited within the body's tissues in the form of microscopic fibers called fibrils. It is not a single disease - at least 20 different forms have been recognized. Each is associated with the deposition of a certain protein and, in fact, the designation of a particular kind of amyloid is based on the type of protein involved. Most often, these components are found normally throughout the body or are produced locally by endocrine glands or other organs. Depending on where they are formed or deposited, some of the organs that can be affected by amyloid are kidney, heart, liver, spleen, nerves, pancreas, or brain. Notably, there are multiple diseases associated with this process; certain of these are acquired while others are inherited. Examples of amyloid-related disorders include Alzheimer's disease, the adult form of diabetes, multiple myeloma and related conditions, chronic infections, and the aging process.
 
Despite the differences in the composition of amyloid, as well as the diseases resulting from its deposition, all forms have common features. One, when tissue that contains this material is stained with the dye Congo red and viewed with a microscope equipped with special polarizing filters, the amyloid exhibits a behavior termed birefringence; that is, when these filters are rotated, there is a striking change in color from deep red to apple green and vice versa.
 
When examined under extremely high magnification with an electron microscope, amyloid appears as rigid, non-branching fibrils. The birefringence and rigidity result from the fact that this material has an unusual structure; namely, these proteins, like silk fibers, assemble as beta-pleated sheets. This feature renders amyloid exceptionally insoluble.
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