Restorative effects of different embryonic cells transplanted into immunodeficient chick embryos.

Cyclophosphamide-treated 18-day chicken embryos were transplanted with cells from different organs of histocompatible embryonic donors. The cells transplanted include cells from the bursa of Fabricius, spleen, bone marrow, thymus or liver of 15- and 18-day embryos and yolk sac cells from 9, 11, 13, 15 and 18-day embryos. To evaluate the reconstitution capacity of the cells transplanted, gain of body weight, weight and microscopic morphology of the lymphoid organs and antibody forming capacity to sheep red blood cells and Brucella were assessed at the age of five weeks. According to all the criteria employed, bursa cells were the only cells capable of a functional and morphological reconstitution of the recipient's humoral immune system. Cells from the embryonic spleen, bone marrow, thymus, liver or yolk sac had no reconstituting effect, indicating that these organs do not harbor precursors for the B-cell lineage. Taken together with other observations, these findings reveal that, as a differentiation site of the B-cell lineage, the bursa of Fabricius precedes the bone marrow during ontogenetic development, and furthermore, that the role of the yolk sac as the first generator of prebursal stem cells must be questioned.