Rabbit anti-serum against a myeloid-cell-specific peroxidase (Mpo) of Xenopus laevis was generated to identify myeloid cells in adult and larval animals. Smears of blood samples from adult hematopoietic organs were co-stained with Mpo and with XL-2, a mouse monoclonal antibody against a leukocyte common antigen. Lymphocytes found in the thymus and spleen were XL-2+Mpo− and granulocytes found in peripheral blood cells and the spleen were XL-2+Mpo+, indicating that double-staining with these two antibodies allowed classification of the leukocyte lineages. Immunohistochemical analysis of larval organs showed that XL-2+Mpo− cells were scattered throughout the liver, whereas XL-2+Mpo+ cells were present mainly in the cortex region. Interestingly, a cluster of XL-2+Mpo+ cells was found in the region of the larval mesonephric rudiment. The ratio of XL-2+Mpo+ cells to XL-2+ cells in the mesonephric region was approximately 80%, which was much higher than that found in other hematopoietic organs. In order to elucidate the embryonic origin of the myeloid cells in the tadpole mesonephros, grafting experiments between X. laevis and X. borealis embryos were performed to trace the X. borealis cells as donor cells. Among the embryonic tissues examined, the tailbud tissue at the early neurula stage contributed greatly to the myeloid cluster in the mesonephric region at stage 48. Therefore, at least four independent origins of the myeloid cell population can be traced in the Xenopus embryo.
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