Developmental rules in the hematopoietic and immune systems of birds: how general are they?

Abstract

Fundamental rules about the development of the hematopoietic and immune systems have been established in birds, some of which are reviewed here. The organ rudiments belonging to this system provide the stroma, that has to be seeded by extrinsic stem cells. This particular developmental pattern makes it possible to create chimeric organs with stromal cells and stem cells from different origins. Grafting the thymic epithelium alone in a young embryo is sufficient to induce tolerance to tissues of the same origin as that of epithelium, even in a xenogeneic combination. Stem cells that seed the rudiments during development are born in the embryo rather than in the yolk sac and are responsible for definitive hematopoiesis. Data in frog and mouse indicate that these conclusions may be valid in other classes of vertebrates.