Nodular renal blastema and metanephric hamartomas in children with urinary tract malformations: a morphologic spectrum of abnormal metanephric differentiation.

In a retrospective review, we identified a variety of abnormal immature and differentiated metanephric tissues including nodular renal blastema and metanephric hamartomas in 12 children with urinary tract malformations with or without renal dysplasia. Four infants also had trisomy. Nodular renal blastema was characterized by small subcapsular clusters of primitive metanephric cells, in some cases with evidence of tubular, papillary, or trabecular differentiation. Metanephric hamartomas included papillary adenomas and tubular hamartomas. Undifferentiated renal blastema was multifocal, but more differentiated blastema usually occurred as isolated nodules. When accompanied by renal dysplasia, blastema tended to show differentiation. One child had a spectrum of lesions with both blastema and more differentiated metanephric hamartomas.