Lineage tracing of FDC progenitors. Ubow⁺⁺-CreERT2 mice were irradiated and reconstituted with WT BM cells to generate chimeric mice with fluorescent stromal cells and nonfluorescent hematopoietic cells. Reconstituted chimeras were treated with tamoxifen, maintained under tamoxifen-free regimen during 1 wk, and injected with an emulsion of CFA/OVA in their ears and rear footpads (left side only). 3 wk later, inflamed popliteal and auricular LNs (B and C) and contralateral LNs (A) were sectioned, stained for CD21/35 (dashed line) and IgD (A and B) or RANK-L (D) expression, and analyzed by confocal microscopy. (C) Quantification of CFP⁺⁺ YFP⁺⁺ CFP⁺YFP⁺ CD21/35⁺ FDC clustering index. Horizontal bar = median. Data are representative of 3 different experiments (2 mice per experiment, 4 LNs analyzed per mouse). Each dot represents one follicle. (E) Comparison of MRC/FDC cluster formation as measured by the clustering index (see Materials and methods section and Fig. S2 for details) in original and simulated data. Each dot represents the cell clustering index of a cell cluster composed of MRCs and FDCs sharing a similar color, in original and Monte Carlo–simulated data. See also Video 1. (F) Auricular/cervical LN sections from Wnt-1Cre Ubow^+/+ mice were stained for CD21/35 and RANK-L expression and analyzed by confocal microscopy. Insets in D and F represent magnified views of juxtaposed MRCs and FDCs sharing a similar color. Data are representative of 3 different experiments (2 mice per experiment, 2 LNs analyzed per mouse). In E and C, a two-tailed Student’s t test was used to determine significance. *, P < 0.08; **, P < 0.01. n.s. = nonsignificant. Bars, 25 µm.