Disorganized bone matrix collagen arrangement in type XII collagen–null mice. (A) Osteoblast morphology was analyzed in cross sections of the diaphysis from femurs of P30 Col12a1^+/+ and Col12a1^−/− mice with H&E staining. Osteoblasts in wild-type bone are localized on the surface of cortical bone (CB), where they are aligned and well organized in an epithelioid manner (arrows). In contrast, Col12a1^−/− osteoblasts (arrows) are poorly aligned and crowded in the periosteum (PO) with a disorganized arrangement associated with the surface of cortical bone. BM, bone marrow. (B) Collagen organization was analyzed in cross sections of the diaphysis from femurs of P14 Col12a1^+/+ and Col12a1^−/− mice. To analyze collagen organization, Sirius red staining with polarized light microscopy was performed. Collagen fibers (arrows) are arranged in parallel and are well organized in the wild-type cortical bone matrix, whereas birefringence is sparse in Col12a1^−/− mice, indicating smaller and/or less organized collagen fibers (asterisks) in the cortical bone. Bars, 50 µm.