Prethymic progenitors from the avian para-aortic mesoderm express GATA-3 and distinct chTcf isoforms but still lack T-cell receptor-gamma rearrangements

Haematopoietic precursors first colonizing the avian embryonic thymus are d erived from the intraembryonic sites located around the dorsal aortae. Thes e intraembryonic precursors have previously been demonstrated to include ce lls that harbour T-cell progenitor capacity and express the Ikaros transcri ption factor, known to be a prerequisite for lymphocyte development. In thi s study, we further evaluated the properties of these prethymic cells. We s how that early intraembryonic cells and prethymic progenitors already expre ss the GATA-3 transcription factor. The chicken homologue of T-cell factor- 1, chTcf, is also detected in cells isolated from the avian para-aortic reg ion. However, these intraembryonic cells retain their T-cell receptor gamma loci in germline configuration. Interestingly, chTcf was found to express different alternatively spliced isoforms during early ontogeny and thymic T -cell development, which indicates developmentally regulated expression of chTcf variants. Taken together, these results demonstrate that, although th e avian prethymic progenitor cells express T-lineage-associated transcripti on factors, they have not yet undergone TCR rearrangements. It is therefore suggested that activation of lineage-associated genes is an early event in the generation of haematopoietic progenitor cells during ontogeny.