Quantification of T-cell progenitors during ontogeny: Thymus colonization depends on blood delivery of progenitors

An in vivo thymus reconstitution assay based on intrathymic injection of he matopoietic progenitors into irradiated chicks was used to determine the nu mber of T-cell progenitors in peripheral blood, paraaortic foci, bone marro w (BM), and spleen during ontogeny. This study allowed us to analyze the re gulation of thymus colonization occurring in three waves during embryogenes is, It confirmed that progenitors of the first wave of thymus colonization originate from the paraaortic foci, whereas progenitors of the second and t he third waves originate from the BM. The analysis of the number of T-cell progenitors indicates that each wave of thymus colonization is correlated w ith a peak number of T-cell progenitors in peripheral blood, whereas they a re almost absent during the periods defined as refractory for colonization. Moreover, injection of T-cell progenitors into the blood circulation showe d that they homed into the thymus without delay during the refractory perio ds. Thus, thymus colonization kinetics depend mainly on the blood delivery of T-cell progenitors during embryogenesis. (C) 1999 by The American Societ y of Hematology.