Studies on the Ontogeny of the Immune Function in Birds

Abstract

Although formal genetics of birds is poorly developed if compared to that of certain mammalian species, the avian model presents certain advantages for immunological studies. The fact that birds possess a specialized organ for B cell differentiation, the bursa of Fabricius, was at the origin of the distinction of the two lymphocytes lineages which differ by their differentiation site and their role in the immune function.1–5 Moreover, the hematogenic hypothesis of the origin of the blood-forming organs was proposed as a result of investigations carried out in birds.6,7 By taking advantage of the possibility of operating on avian embryos during the critical period of organogene-sis, it was thereafter demonstrated that all the hematopoietic organs have to be seeded by stem cells of extrinsic origin during embryogenesis to differentiate and become functional.8–10 Thus, the decisive steps of thymic ontogenesis have been precisely defined in two closely related species of birds, the chick (Gallus gallus) and the Japanese quail (Coturnix coturnix japonica), both of which belong to the same family (the Phasi-anidae). The interest of working on these particular species resides in the cell marking system based on the unique structure of the interphase nucleus of all embryonic and adult cell types of the quail.11,12 Furthermore, development of the avian embryo allows foreign tissues to be grafted in the embryo in ovo, either between quail and chick or between chickens differing by the major histocompatibility complex (MHC), prior to the onset of immune system differentiation. One can, by this means, construct viable birds that are able to hatch and study the immunological status of the graft after birth when the host–s immune function has reached maturity.