Is the yolk sac the primary origin of lymphoid stem cells?

Abstract

Two-day-old chick embryos were transplanted on age-matched histocompatible yolk sac resulting in the development of yolk sacembryo chimeras. Eleven of these grew through the hatching. Difference in the sex and in the IgG allotype between the embyro and the yolk sac was utilized to study contribution of these two components to the lymphoid cell development. At 5-7 weeks of age the chimeras proved to be completely normal according to IgM and IgG antibody production against HGG and Brucella abortus as well as to lymphocyte responses against PHA and Con-A. For the sex chromosome analysis bursa cells and specifically stimulated T-and B-lymphocytes were used. The latter was achieved by stimulating thymus, spleen, and bone marrow cells in vitro with Con-A and anti-Ig. Only 4 of 1498 cells analyzed belonged to the sex opposite to that of the bird as diagnosed from the gonads. Among the chimeras four were also marked for the IgG allotypes. At the age of five weeks all four showed serum IgG of the embryo allotype (G1(a)) and none of the yolk sac type (G1(b)). These results, based on the chromosome analysis of bursa cells and of specifically stimulated T- and B-lymphocytes in thumus, spleen, and bone marrow as well as on IgG allotypes, indicate that lymphoid stem cells in the chicken are primarily derived rather from an intra-embryonic source than from the yolk sac.