Development of the compartments of the immune system in the avian embryo

Because of the easily traceable quail-chick marker, the early development of the immune system, in particular the connections between the differentiating organs, has been extensively studied in birds. These earlier studies clarified the origin, movements and potentialities of progenitors, and the colonization of primary lymphoid organs. With the advent of an array of similar markers in mammals, it will now be possible to identify the interactions between the stromal components of lymphoid organs and the progenitors, which give rise to the effector cells of the immune system. Rules established over the last 20 years, such as extrinsic origin of stem cells, periodic colonization of the thymus, unique colonization of the bursal rudiment, are now extended by new cellular and molecular acquisitions. Relationships between vasculogenesis, angiogenesis and hemopoiesis, acquisition of xenogeneic tolerance by thymic epithelium grafting, avian-specific strategy of immunoglobulin diversification, and identification of thymotaxin, the polypeptide responsible for progenitor attraction to the thymus, are reviewed.