INTRAEMBRYONIC HEMATOPOIETIC-CELLS AND EARLY T-CELL DEVELOPMENT

T cell precursors in the chick embryo have been localized into the int raembryonic mesenchyme (IEM) and into the para-aortic region before th e first wave of the thymic colonization on embryonic day (ED) 6,5-8. T he cell surface markers of avian prethymic stem cells are not known. I t is also not known whether these precursor cells are already committe d to the T cell lineage before their thymic colonization. In 7-day-old chick embryos Ov(+) cells were found in the para-aortic region. Also the endothelial cells of the embryonic dorsal aorta were positively st ained. Ov antigen might represent a very primitive marker for precurso r cells having the potentiality to differentiate both to haemopoietic and endothelial cells. Scattered CD45(+) cells were observed in the sa me para-aortic area as in many haemopoietic areas in the loose embryon ic mesenchymal tissues. CD8 alpha (MoAb 3-298) expressing haemopoietic cells were detected before thymic colonization on ED6. In how cytomet ric analysis of IEM precursors Ov, CD45 and CD8 alpha expressing cells seemed to form distinct subsets suggesting heterogeneity of these hae mopoietic cells.