T-CELL PRECURSOR MIGRATION TOWARDS BETA-2-MICROGLOBULIN IS INVOLVED IN THYMUS COLONIZATION OF CHICKEN EMBRYOS

β2-microglobulin (β2m) attracts hemopoietic precursors from chicken bone marrow cells in vitro. The cell population responding to β2m increases during the second period of thymus colonization, which takes place at days 12-14 of incubation. The precursors from 13.5 day old embryos were isolated after migration towards β2m in vitro and shown to be able to colonize a 13 day old thymus in ovo, where they subsequently acquire thymocyte markers. In contrast these β2m responsive precursors did not colonize embryonic bursa, i.e. differentiate into B lymphocytes. During chicken embryogenesis, peaks of β2m transcripts and of free β2m synthesis can only be detected in the thymus. The peak of free β2m synthesis in the thymus and the increase of β2m responding bone marrow cells both occur concomitantly with the second wave of thymus colonization in chicken embryo, facts which suggest that β2m mediated chemotaxis is involved in the second wave.