Kinetics of chicken embryonic thymocyte development in ovo and in organ culture

Thymocyte development was monitored in an embryonic thymus organ culture system to establish a model in the chicken in which the functional nature of the thymic microenvironment could be assessed. Thymus lobes were removed from 10‐day‐old embryos and cultured for 2–10 days. Cell yield increased to a maximum in 4–8 days of culture with a corresponding decrease in average cell size. An initial thymocyte population of predominantly CD3⁻CD4⁻CD8⁻ cells gave rise to all CD3/CD4/CD8‐defined subpopulations in vitro, maintaining high levels of CD3⁻CD4⁻CD8⁺ and CD3⁺CD4⁻CD8⁺ cells and a low representation of CD3⁻CD4⁺CD8⁻, CD3⁺CD4⁺CD8⁻, CD3⁻CD4⁺CD8⁻ and CD3⁺CD4⁺CD8⁺ thymocytes. This is the first observation of a CD3⁻CD4⁺CD8⁻ population in the chicken. Developmental kinetics of CD3⁺ cells were similar to that in the embryo, suggesting that the in vitro environment is sufficient to promote and maintain thymocyte maturation. Thymocytes of both the γδ and αβ T cell receptor (TcR) lineages developed in that order, confirming in ovo data and the lineage potential of the first wave of thymocyte precursors. One unusual finding was a relative accumulation of γδ TcR⁺ thymocytes in culture, incorporating all CD4/CD8 subsets, including a previously undetected population, CD4⁺CD8⁻. This may indicate a favorable developmental environment or simply a lack of normal cellular emigration. A detailed comparison with T cell development in the embryo demonstrated that the chicken thymus organ culture system reflects thymic events in ovo during a limited time period and thus should prove useful in the identification of functionally relevant thymic molecules.