Tissue recombination of mES cells with rat UGM. Rat UGM cells (300,000) were recombined with (A) 500, (B) 1000, (C) 1500, and (D) 5000 mES cells and xenografted under the renal capsule of nude mice. The resulting grafts after 6 weeks varied in gross size (inset) and histology (A-D). The recombinant grafts with UGM and the minimum 1500 mES developed teratomas which contained 10-20% prostatic structures. Positive immunohistology staining for (E) FoxA1, (F) p63, (G) AR, and (H) mDLP secreted protein supported our interpretation of prostatic differentiation. The sections were nuclear counterstained with hematoxylin (blue).