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Pathologic accumulations of glycogen occur in the tissues of patients with diabetes mellitus in whom the normal cellular uptake of glucose is impaired. Excessive storage of glycogen is also found in genetic diseases in which there is absence of one or another of the enzymes that constitute the Embden-Meyerhof glycolytic pathway, or when an abnormally structured glycogen that cannot be degraded by glycolytic enzymes is synthesized. Intracellular accumulations of glycogen impart a vacuolated appearance to the cytoplasm. Since accumulations of glycogen morphologically resemble those of water (hydropic swelling) and fat (triglyceride under the light microscope), one of the following tests is necessary to establish that the deposit is glycogen: (1) application of the periodic acid-Schiff test in which glycogen is stained reddish purple (tissues previously digested with diastase serve as negative controls) or (2) quantitative analysis of the glycogen content of affected tissues.
Massive intracellular deposits of glycogen occurring in glycogen storage disease cause affected organs such as the liver, kidney or heart to be greatly enlarged and their function to be ultimately compromised. In diabetes mellitus, glycogen deposits are encountered in epithelium of the distal segment of the proximal convoluted renal tubules and the descending loop of Henle, hepaocytes, beta cells of pancreatic islets, and cardiac muscle. Accumulation in the kidney occurs when the degree of glycosuria leads to levels of glucose in the glomerular filtrate that exceed the rate at which glucose is reabsorbed by epithelial cells of the renal tubules (tubular mass). Excessive intracellular deposits of glycogen lipids, glycolipids, mucolipids, and mucopolysaccharides develop in patients with genetic disorders characterized by lysosomes with specific enzyme deficiencies that render them incapable of hydrolyzing them. Abnormal metabolites accumulate in neurons of the centrals nervous system, as well as parenchymal cells of the liver, kidney, and heart, and cells of the reticuloendothelial system such that massive enlargement of the spleen and to a lesser degree the lymph nodes and liver occurs. As in the other storage diseases attributable to gene defects, involvement may become so severe that it is incompatible with life. These and similar diseases are considered in greater detail in a subsequent section dealing with inborn errors of metabolism.
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Introduction of Pathology An outline of Diagnostic Techniques available in Pathology Diagram showing Structural Changes in Reversible and Irreversible Cell Injury |