Lymphoid stem cells in the intraembryonic mesenchyme of the chicken

Chromosomally marked cells from the 7-day intraembryonic mesenchyme were transplanted into 14-day-old irradiated chick embryos. At the age of 6 weeks donor-derived T and B lymphocytes were shown to be present in the thymus, spleen and bone marrow, indicating that cells in the 7-day intraembryonic mesenchyme are capable of developing into functional T and B lymphocytes. In addition to the sex chromosome marker, IgG allotype was used as a marker; the results demonstrate that cells from intraembryonic haemopoietic sites develop into mature IgG-producing cells. In similar experiments, the 7-day yolk sac also proved to contain lymphoid stem cells. Since lymphoid cell progenitors are not present in the 2-day yolk sac, as has been shown previously in the yolk sac-embryo chimaeras and since circulation is established from day 2 of incubation onwards, lymphoid stem cells present in the 7-day yolk sac are most likely secondary immigrants originating in the intraembryonic mesenchyme.