(A) Time-lapse live imaging identifies prolonged cell-cell contacts between runx1+23:EGFP+ HSPCs and mpeg1:mCherry+ primitive macrophages that involve the exchange of fluorescent material. (B) About 20 to 30% of HSPCs interact with macrophages in the CHT at any one time from 56 to 106 hpf. Data are means ± SD. (C) High-resolution tracking of individual runx1+23:mCherry+ cells over several hours in the CHT reveals that most HSPCs eventually make sustained contact with macrophages (>5 min). Data are means ± SD. (D) HSPCs frequently complete a cell division shortly after macrophage interactions. About 81% of HSPC divisions occur within 30 min of a macrophage interaction. (E) About 65% of Fucci+ HSPCs in S, G2, and M phases interact with macrophages at any one time, as compared to less than 20% of Fucci– HSPCs. Data are means ± SD. Data were analyzed by unpaired Student’s t test; ****P < 0.0001.