Early light microscopic changes in chronic progressive nephrosis in several strains of aging laboratory rats.

The kidneys of 441 full-lifespan rats of Wistar (WAG/Rij) and Brown Norway (BN/Bi) strains and a Sprague-Dawley stock were evaluated histologically for chronic progressive nephrosis. Age- and strain-related susceptibility to chronic progressive nephrosis, in terms of degree of lifetime pathological change, was characterized for male and female WAG/Rij and BN/Bi and female Sprague-Dawley rats. Criteria for minimal chronic progressive nephrosis included thickening of the basement membrane of a glomerular capsule, cast formation, and atrophy of a segment of a proximal convolution with thickened basement membrane. The focal and segmental changes in increasing numbers of nephrons were criteria for severity of the disease. Sprague-Dawley rats showed comparatively rapid development of chronic progressive nephrosis during their second year of life, in contrast to the BN/Bi rats that were affected only slightly prior to 28 and 31 months of age, after which the disease progressed slowly. WAG/Rij rats appeared to assume an intermediate position between the other strains, the latter probably representing the extremes in the expression of this disease.