Ontogeny of ramified CD45 cells in chicken embryo and their contribution to bursal secretory dendritic cells

Embryonic tissues contain highly ramified stellate-shaped cells expressing CD45 and MHC II antigens but their origin and immunophenotype are unknown. Using staged avian embryos and cell-type-specific antibodies, we establish a detailed spatiotemporal ontogeny of cells that express CD45, the earliest marker of hematopoietic stem cells in the chick. CD45 immunostaining marks three distinct embryonic cell populations: round, ramified and amoeboid cells. The round and ramified CD45+ cells appear first in yolk-sac blood islands before the onset of circulation. A subpopulation of round cells co-expresses the thrombocyte-specific CD51/CD61 antigen. Amoeboid cells express macrophage-specific antigens and frequently occur in regions of apoptosis. Ramified cells are distributed uniformly in the embryonic mesenchyme, colonize lymphoid and non-lymphoid organs and later express MHC II. To study the origin of CD45+ cells, 2-day-old chick embryos were ablated from the yolk sac before the establishment of circulation and incubated for 2–5 days. Large numbers of CD45+MHC II+ ramified cells differentiated in the yolk sac. Yolk-sac chimeras were generated by grafting embryos into GFP-expressing de-embryonated yolk sacs. GFP/CD45 co-expressing ramified and amoeboid cells colonized all organ primordia in the donor embryo. We also recombined GFP+ yolk sac with the bursa of Fabricius and found ramified GFP+CD45+ cells in the bursa where they differentiated into dendritic cells. Thus, CD45 cells are first present in the yolk sac during primitive hematopoiesis and then migrate from the extra-embryonic yolk sac to give rise to cells throughout all organ primordia, including dendritic cells in the bursa of Fabricius.