Progressive thymic epithelium phenotypes in Foxn1^lacZ mutants. (A-D) Cryosections were stained for Keratin 8 (green) and Keratin 5 (red). Scale bar: 200 μm. Arrowhead, perivascular space; *, cystic structure. (A) Two-week +/lacZ, +/nu, and lacZ/lacZ thymi showed organized cortico-medullary architecture; lacZ/nu thymus also show a breakdown of the cortico-medullary junction (CMJ). (B) At 3 weeks, lacZ/lacZ thymus shows CMJ degeneration, and there was a more severe phenotype in lacZ/nu thymus, with no organized cortical or medullary regions. (C) At 6 weeks, lacZ/lacZ and lacZ/nu thymi had an even more severe defect compared with 3 weeks. (D) A 20-month-old +/+ thymus is shown as a normal involution control. (E,F) Frozen thymus sections stained with anti–Keratin 14 (red) and UEA-1 (green). Scale bar: 200 μm. Both the 3-week lacZ/lacZ and lacZ/nu (E) and the 22-month-old +/+ thymus (F) have fewer UEA-1⁺ mTECs within K14⁺ medullary regions.