ProB and preB cell migratory behavior in vivo. (A) Distribution of GFP⁺ proB cells (green, top left) and of preB cells (green, bottom left) in BM calvaria of lethally irradiated Cxcl12^dsRed/+ mice reconstituted with Rag1^GFP/GFP (top panels) or with Rag1^GFP/GFP Igh^3-83/+ (bottom panels) BM cells. Movement of GFP⁺ B cells tracked for 30 min in vivo by IVM. Colored lines (right panels) represent cell trajectories. Scale bar, 50 µm. (B) Median velocity (micrometers per minute). Each circle represents a single cell analyzed. (C) Mean motility coefficient of proB and preB cells. Cell displacement from starting coordinates is plotted against time (minutes). Lines represent the mean motility coefficient from three separate videos per mouse. (D) Time (minutes) spent in direct contact with CXCL12⁺ cells over 30 min. (E) Average time (minutes) proB and preB cells spent in contact with individual CXCL12⁺ cells. Data are from Video 1. Circles represent single cells analyzed. ****, P < 0.00005 by unpaired Student’s t test. Data in all panels are representative of three independent experiments.