Pathological accumulations of glycogen were studied in the kidney tubular epithelium in untreated STZ-induced diabetic rats of 50 days diabetes duration. Blood glucose concentrations were ∼17 mM, and the animals had no ketonuria. At the termination of the experiment, the kidneys were perfusion-fixed, and serial sections were cut from the renal capsule to the tip of the papilla and stained with toluidine blue and periodic acid Schiff. By tracing tubular profiles from section to section in a light microscope, the outlines of nephrons were reconstructed, and abnormal glycogen accumulations were mapped in accordance with the outlines. The exact segmental localization, character, and extension of the glycogen accumulations were determined. The predominant location of the pathological glycogen accumulations was in the thick ascending limb of Henle's loop. Dot-shaped and diffuse-appearing glycogen accumulations were discretely distributed throughout the segment, and large confluent cytoplasmic accumulations of glycogen were also present. On a continuous basis, glycogen was present only in the cortical thick ascending limb of Henle's loop and the macula densa segment excluding the macula densa cells. In the distal convoluted tubule and the cortical collecting duct system, scattered dot-shaped and diffuse glycogen accumulations were discretely distributed. Furthermore, glycogen appeared as confluent, cytoplasmic accumulations in the initial part of the descending thin limb of Henle's loop. In addition, glycogen accumulations were found in nuclei within a narrow stripe of the outer stripe of the outer medulla in the medullary thick ascending limb of Henle's loop.

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