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Before their hematopoietic stem cell origin was recognized, chicken thymocytes were thought to be derived from epithelial precursors in the thymus that could be converted into lymphocytes under the influence of surrounding mesenchymal tissue (Auerbach 1961). Evidence obtained later indicated that embryonic yolk sac contains the precursors of lymphoid cells (Moore and Owen 1967). Yolk sac stem cells at the first and second day of embryonation (E1–2) were then shown to be derived from the embryo itself (Martin et al. 1978). Further studies indicated the presence of pluripotent stem cells in the region of the thoracic aorta as early as E4, a few days before some stem cells can be found in the spleen. These studies, conducted in chick-quail chimeras, indicated an aortic (E4) followed by a paraortic (E6) origin of precursors of thymocytes, B cells and myeloid cells. It was suggested that embryonic stem cells native to the aortic region migrate via the circulation and colonize the spleen, yolk sac, and finally the bone marrow. However, recent studies have shown that the stem cells produced in these different organs during ontogeny may correspond to different populations generated separately since they do not present identical properties (see chapter by Dieterlen). |
