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Is yolk sac the primary origin of lymphoid stem cells?

Development of lymphoid organs in the chick embryo is dependent on colonization by extrinsic stem cells which have been presumed to originate in the yolk sac blood islands. However, we have recently presented evidence to indicate that stem cells originally formed in the yolk sac play little, if any, role in the colonization of lymphoid organ rudiments. Our evidence was based on the use of chick yolk sac-embryo chimeras (Lassila et al., Nature, in press). These results do not support the yolk sac origin of lymphoid stem cells in the chicken. In the present study we have been successful to grow the 'sex chimeras' through the hatching. For the chromosome analysis bursa cells and specifically stimulated B and T lymphocytes were used. The latter was achieved by stimulating thymus, spleen and bone marrow cells in vitro with anti-Ig and Con A. Only four out of 1510 cells analyzed belonged to the sex opposite to that of the birds as diagnosed from the gonads; eleven 5-8 week-old yolk sac-embryo chimeras were used for these analyses. Among the chimeras four birds were also marked for the IgG allotypes. At the age of six weeks all four showed serum IgG of the embryo allotype (Gla) and none of the yolk sac type (Glb). These results, based on the chromosome analysis of bursa cells and of specifically stimulated B and T lymphocytes in thymus, spleen and bone marrow as well as on IgG allotypes, indicate that lymphoid stem cells in the chicken are primarily derived rather from an intraembryonic source than from the yolk sac.