Cellular and molecular characterization of fetal thymus recruitment. (A) TSCs dissociated from dGuo-treated E14.5 thymus lobes were stained for I-A, CD45, and EpCAM (top panels). Numbers indicate the frequency of cells within each quadrant. TSCs were sorted into CD45^–I-A⁺ cells and CD45^–I-A^–/low cells (bottom panels). (B) Representative images at indicated time points of the edge of a drop of fetal thymocytes (1 × 10⁵) in the presence of reaggregate of CD45^–I-A⁺ TSCs (top panels) and CD45^–I-A^–/low TSCs (bottom panels). Note that many fibroblast-like spikes spread out of the reaggregate of CD45^–I-A^–/low TSCs during culture. Original magnification is as indicated in Figure 1B. (C) Means and SEs of the numbers of E14.5 fetal thymocytes that moved out of the cell spot (□) and that reached the reaggregate of indicated cells (▨) were determined in visualized field. Numbers of independent experiments are also indicated. CD45^–I-A^–/low TSCs attracted a significantly smaller number of cells than CD45^–I-A⁺ TSCs (P < .05). (D) E14.5 fetal thymocytes were treated with phosphate-buffered saline (None) or 100 ng/mL PTX at 37°C for 2 hours. Means and SEs of the numbers of cells that moved out of the cell spot (□) and that reached the dGuo-treated fetal thymus (▨) were determined in visualized field. Numbers of independent experiments are also indicated. PTX significantly inhibited the numbers of cells that were attracted to the fetal thymus (P < .001).