Slide 8 demonstrates the relative frequency with which certain glomerular diseases present as either the nephrotic syndrome or the nephritic syndrome. Some diseases, for example, minimal change glomerulopathy and membranous glomerulopathy, very frequently cause nephrotic syndrome without substantial nephritic features. Other diseases, especially the so-called proliferative glomerulonephritides, which usually have a lot of leukocyte infiltration and lots of inflammatory injury to the integrity of the glomerulus, usually cause the nephritic (glomerulonephritic) syndrome.
Slide 9 shows a histologic feature that is found in any patient with nephrotic range proteinuria no matter what the cause, i.e., substantial resorption into the proximal tubular epithelial cells of proteins and lipoproteins that are spilled into the urine. In this trichrome-stained section the droplets are red. They would be black with a silver stain, and purple with a PAS stain. The cytoplasm of these engorged epithelial cells sometimes sloughs into the lumen as little chunks of cytoplasm containing droplets of lipoproteins and proteins. Slide 10 (standard light microscopy and polarized light microscopy) shows the appearance of these in the urine as so-called oval fat bodies. Oval fat bodies can be seen quite nicely with polarized microscopy because of the birefringence of the lipid, which produces maltese cross configurations. Oval fat bodies are markers for nephrotic range proteinuria but not for any particular disease. These lipid droplets also can become incorporated into casts (slide 11), i.e., fatty casts. In summary, fatty casts and oval fat bodies are characteristic of the nephrotic syndrome and derived from epithelial cells that have engorged themselves with the lipoproteins and proteins spilled during nephrosis.
