In situ hybridization shows that cells within the vessel walls and biliary ducts are of human origin. Combined in situ hybridization and immunohistochemistry were performed on liver sections from sheep that underwent transplantation using a probe specific for the human Alu sequence and an antibody specific for human CD45. (A) Control (no transplantation) sheep. (B-C) Two different sections show numerous cell nuclei within the vessel walls and biliary ducts stained dark brown, demonstrating that they are of human origin. None of these cells exhibit red staining for CD45.