Early development of embryonic thymus in MafB-deficient mice. (A-C) Sagittal sections of indicated mice at E12.5 (A) or E13.5 (B) were 2-color–stained for CD45 (green) and keratin (red). Sagittal E13.5 sections of frozen embryos from indicated mice were also 2-color–stained for FoxN1 (red) and keratin (blue; C). Scale bars indicate 50 μm in panel A and 150 μm in panels B and C. (D-F) Means and standard errors (n = 10 for panel D, n = 4 for panel E, and n = 2 for panel F) of the numbers of CD45⁺ leukocytes (D,E) or FoxN1⁺ cells (F) in keratin⁺ thymic areas per section of E12.5 (D) and E13.5 (E,F) embryos are shown. ***P < .001; **P < .01; N.S., not significant.