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The intracoelomic route for in utero hematopoietic stem cell transplantation has been evaluated in pre-immune fetal sheep and the engraftment characteristics defined. Twelve ovine fetuses (gestational ages: 40-45 days) received intracoelomic transplants of human CD3-depleted (50 x 10(6) per lamb) or CD34-selected (1-2 x 10(5) per lamb) cord blood hematopoietic stem cells. Engraftment was evaluated from cell suspension of the liver, spleen, bone marrow and thymus by flow cytometry, cloning assays and polymerase chain reaction (PCR) analysis for human beta(2)-microglobulin gene. The engraftment of liver samples was also evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR), fluorescent in situ hybridization (FISH) and immunohistochemistry. Four fetuses (33%) aborted shortly after intracoelomic transplantation and were not evaluable for engraftment. Engraftment was detected in 4 fetuses obtained from cesarean delivery on day 70 after transplantation of CD3-depleted cord blood cells. The degree of engraftment in these 4 fetuses ranged from 6 to 22% in the different organs (as revealed by antigenic analysis of human CD45 with flow cytometry). Three fetuses obtained after cesarean section at 102 (No. 435184) and 105 (Nos 915293, 037568) days and 1 fetus delivered at term, which received CD34-selected cord blood cells, had human engraftment with 10, 32, 20 and 10% CD45+ cells in bone marrow, respectively. A further check of human chimerism was done at 1 year after birth of the fetus delivered at term and 7.6% of bone marrow chimerism was detected. In 6 out of 8 fetuses evaluable for human engraftment, chimerism was confirmed by PCR analysis for human beta(2)-microglobulin which also identified human cells in brain, spinal cord, heart, lung and skeletal muscle. On liver samples, FISH and RT-PCR confirmed the xenograft of human cells and the immunohistochemical analysis detected human markers of hematopoietic and hepatic lineage of differentiation. This preli