Alport's syndrome: specificity and pathogenesis of glomerular basement membrane alterations

In Alport's syndrome (AS) thinning and splitting of the glomerular basement membrane (GBM) are assumed to be characteristic ultrastructural alterations. Both lesions are, however, non-specific because they can occur in other glomerulopathies. In addition, splitting may be found in non-glomerular structures. It should be emphasized that the characteristic lesion in AS is a result of the widespread combination of thin and split GBM in the same biopsy specimen. In our opinion the basic lesion is the thin GBM, which is characterized by a lamina densa (measuring 50-150 nm in thickness) which may begin to split as a result of focal detachment of podocyte pedicles (spacing) and repeated subepithelial deposition of new lamina densa layers. Splitting thus appears to be a secondary lesion. Thinning of GBM may represent a persistent embryonal status of the lamina densa and may thus be the result of a development defect. This assumption is supported by the findings of fetal-like glomeruli and small capillary loops in AS.