Intralobar nephroblastematosis in a nine-week-old Wistar rat.

Intralobar nephroblastematosis (ILNB) is a precursor lesion to the development of nephroblastoma (NB) in rats. Unilateral ILNB was observed in the kidney of a nine-week-old female Wistar rat (Crl:WI) from a short-term toxicity study. Clinical pathology and urinalysis did not reveal altered renal function. This microscopic, unencapsulated lesion consisted of basophilic sheets of blastemal cells that did not include a prominent mesenchymal component. These cells expanded in the interstitium, which trapped and compressed few normal renal tubules. The blastemal cells moderately differentiated to form rosettes, primitive tubules, and a glomeruloid body. Multifocally, the lumen of primitive tubules contained eosinophilic secretions with basophilic material in the center. The diagnostic criteria used were compared and differentiated with renal dysplasia, nephrogenic rest, NB in rats, and with that of the identical lesion in children.