Interstitial
fibrosis is a powerful pejorative predictor of progression of nephropathies in a variety of
chronic renal diseases. It is characterized by the depletion of kidney cells and their replacement by extracellular matrix, in particular, type-I
fibrillar collagen, a
protein scarce in normal interstitium. However, assessment of
fibrosis remains a challenge in research and clinical pathology. We develop a novel methodology based on second harmonic generation (SHG) microscopy, and we image
collagen fibers in human and mouse unstained kidneys. We take into account the variability in renal shape, and we develop automated image processing for quantitative scoring of thick murine tissues. This approach allows quantitative 3-D imaging of interstitial
fibrosis and arterial remodeling with high accuracy. Moreover, SHG microscopy helps to raise pathophysiological questions. First, imaging of a large volume within a mouse kidney shows that progression of
fibrosis is a heterogeneous process throughout the different renal compartments. Second, SHG from
fibrillar collagens does not overlap with the glomerular tuft, despite patent clinical and experimental glomerulosclerosis. Since glomerulosclerosis involves SHG-silent
nonfibrillar collagens, our work supports pathophysiological differences between interstitial
fibrosis and glomerulosclerosis, a clearly nonfibrotic process.