Neoplastic cells proliferate inside tumor tubules creating a "glomeruloid" appearance.
Nuclei may show folds, indistinct nucleoli and absent or rare mitosis.
Degenerative changes including psammoma calcifications may be present.
Immunohistochemistry: Nuclear WT1+ (or Wilm's tumor 1), AMACR+ and CD57+.
Wilm's Tumor: Similar primitive appearing cells but is more atypical with mitosis, is triphasic (also has mesenchymal and blastemal components) and is CD57-.
Papillary RCC, type 1 or basophilic type: More cytoplasm than MA, usually WT1- or CD57-, and has +7, +17 and -Y (cytogenetics usually not necessary).