|1) Cystic Partially Differentiated
This is a well demarcated lesion composed of cysts often with a hobnail epithelial lining, and with Wilms' tumour-like tissue in their walls.
This lesion is adequately treated by excision alone in children, and is distinguished from cystic Wilms' tumour by the absence of any solid area resembling nephroblastoma.
|2) Metanephric (Nephrogenic)
This tumour arises deep in the kidney.
The histologic appearance is characterized by a marked proliferation of spindled mesenchymal cells resembling the classical type of congenital mesoblastic nephroma, encasing nodules of embryonal epithelium similar to the hyperplastic nephrogenic rests (nephroblastomatosis) usually associated with Wilms' tumor.
The mesenchymal component consists of a fascicular proliferation of tightly interlaced, uniform, benign-appearing spindled cells that immunostain for vimentin and fibronectin, but not desmin or actin.
The epithelial component consists of discrete islands of blastemal cells that are partially or fully differentiated toward tubular, tubulopapillary, or papillary structures.
Numerous psammoma bodies are present.
Embryonal epithelium immunostains for cytokeratin but not epithelial membrane antigen.
The overall histologic appearance of the mesenchymal and epithelial components is benign.
3) Metanephric Adenoma:
This tumour is composed exclusively of epithelium with characteristic small dark cells arranged in tiny tubules and papillary formations also often with psammoma bodies.
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