Exposure to the thymic microenvironment renders pro–B cells unresponsive to B-lymphopoietic stimuli.

(A) Cells harvested from FTOCs 2 days after seeding lobes with 5 × 10⁴ CD45R⁺sIgM⁻ BM cells can still establish long-term cultures on S17 stroma. (B) CD45R⁺sIgM⁻ cells can no longer establish long-term BM cultures following 7 days of culture in FTOCs. Original magnification, × 100.