Answer: Renal Dysplasia

Histology: This kidney demonstrates marked lobar disorganization. There is prominent medullary fibrosis with concentric cuffing of collecting ducts by bland spindled cells. (so-called “primitive ducts”). Also present are islands of cartilage. The subcapsular glomeruli have a somewhat immature look, but collections of blastemal- or primitive tubular cells characteristic of nephrogenic rests are absent.

Discussion: Metanephric stromal tumor is a spindle cell neoplasm composed of bland fibroblastic cells which tend to concentrically cuff entrapped renal tubules, similar to the pattern seen in renal dysplasia. Metanephric stromal tumor differs from dysplasia in that it forms a mass lesion and often demonstrates intratumoral angiodysplasia and heterologous differentiation. Nephrogenic rests are collections of primitive renal elements that are found in less than 1% of neonatal autopsies, but found in 4% of dysplastic kidneys. Congenital mesoblastic nephroma is an infantile fibroblastic neoplasm that may either resemble a fibromatosis (classic type) or a fibrosarcoma (cellular type).

Renal dysplasia is a manifestation of fetal urinary outflow obstruction, and therefore implies obstruction of urinary outflow in utero. Renal obstruction early on in gestation yields severe dysplasia, whereas late onset obstruction yields dysplasia affecting only the most superficial glomeruli.