Wnt4 controls fetal thymic size through regulation of TEC numbers. (A) TEC numbers in E15.5 and E18.5 Wnt4^+/+ and Wnt4^−/− thymi dissociated by enzymatic digestions and analyzed by flow cytometry. (B) Thymocyte to TEC ratio in E15.5 and E18.5 thymi. (C) Hematoxylin- and eosin-stained paraffin sections of thymi from Wnt4^+/+ and Wnt4^−/− mice at E18.5. Original magnification ×40. Higher magnification views of selected regions (black boxes) are shown on the right. m indicates medulla; c, cortex. (D) Frozen thymic sections from Wnt4^+/+ and Wnt4^−/− mice stained with anti-K8 (i-viii), anti-K5 (i,v), anti-K 14 (iii,vii), and anti-β5t (iv,viii). Higher magnification views of selected regions (white boxes) are shown for K8/K5-stained thymic sections (ii,vi). 10×/0.3 NA (i,iii,v,vii), 40×/1.3 NA oil objective (iv,viii), and 63×/1.4 NA oil objective (ii,vi). (E) Quantitative analysis of β5t and K5 staining (left panel) and the area occupied by medullary regions (right panel) per lobe of E18.5 Wnt4^+/+ and Wnt4^−/− thymi. Histograms represent mean ± SEM of 8-10 mice (*P < .05, **P < .01, ***P < .005).