Cyclophosphamide-treated 18-day-old chick embryos were transplanted with cells from 7-day intraembryonic mesenchyme; the recipients and donors were identical at the major histocompatibility locus. At the age of 35 days, the cell recipients were studied to determine the reconstitution capacity of the transplanted cells. The transplantation resulted in a complete restoration of IgM and IgG class antibody production against human gammaglobulin and Brucella abortus, and of microscopic morphology of the bursa of Fabricius and of the germinal center formation in the spleen. These findings demonstrate that 7-day intraembryonic mesenchyme of the chick embryo harbor prebursal stem cells. These findings confirm our previous observations in the yolk sacembryo chimeras indicating that lymphoid stem cells originate in the intraembryonic hematopoietic sites.

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This study was supported by contracts with the United States National Cancer Institute (NO1-CB-74177) and the Association of Finnish Life Insurance Companies, and by a grant from the Lääke Oy Research Foundation, and Emil Aaltonen Foundation.

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