THE PATHOGENESIS OF THE GRAFT‐VERSUS‐HOST REACTION IN CHICKEN EMBRYOS. PATHOLOGICAL CHANGES IN THE YOLK SAC, THYMUS, BONE MARROW AND BURSA OF FABRICIUS
Summary: The effect of a graft‐versus‐host reaction on the development of the yolk sac, bursa of Fabricius, thymus and tome marrow was examined in chicken embryos inoculated at different stages of development with blood from an allogeneic adult donor. Haemopoietic tissue of the yolk sac folds showed depletion of stem cells at each stage investigated but it was particularly severe in very young embryos. The effect of a graft‐versus‐host reaction on intraembryonic haemopoietic tissue was either aplasia or the appearance of proliferative lesions, depending on the age of the recipient and the developmental stage of the particular organ. Lymphopoiesis in the thymus was severely depressed in embryos inoculated at day 5 or day 6, and follicles in the bursa from embryos inoculated at day 8 to day 14 were poorly developed. Proliferative lesions first appeared in the thymus when embryos were inoculated at day 6 or day 8. They first developed in the bone marrow of embryos inoculated at day 10, but were not found in the bursa of embryos inoculated before day 14. The time when proliferative lesions could first be induced in these organs closely correlated with the time at which they were populated with yolk‐sac‐derived stem cells. The nature of a graft‐versus‐host reaction in chicken embryos therefore appears to depend on the development and maturation of its haemopoietic tissues.