Additional Value of Second Harmonic Generation Microscopy in the Diagnosis of Feline Collagen Type III Glomerulopathy

A 1.5-year-old neutered female Domestic Shorthair cat was euthanized after the diagnosis of end-stage protein-losing nephropathy associated with the onset of nephrotic syndrome. At necropsy, both kidneys were diffusely pale and swollen with a granular cortex. Histologically, glomeruli had diffuse global mesangial and capillary wall expansion by homogeneous pale eosinophilic material. This material was Congo red negative, blue with Masson's trichrome stain, weakly positive with periodic acid-Schiff stain, bright red with Picrosirius red and birefringent under polarized light. Transmission electron microscopy and second harmonic generation (SHG) microscopy revealed mesangial and subendothelial collagen fibril deposition. Type III collagen deposition was confirmed by immunohistochemistry. This study provides an original and complete description of feline collagen type III glomerulopathy and emphasizes the possibility of directly diagnosing glomerular collagen deposition on unstained slides through SHG microscopy.