Immunohistochemical analysis of M33-KO spleens. The expression of marker proteins was compared among wild-type (A,D,G), M33-KO (B,E,H), and Ad4BP/SF1-KO (C,F,I) spleens at 18.5 days after coitus. Frozen sections were stained with antibodies against type-IV collagen (A-C), laminin (D-F), and vimentin (G-I). Closed arrowheads indicate trabeculae. An arrow indicates a vascular structure. Open arrowheads indicate cells assembled around the vascular structures. Scale bar, 100 μm.