Altered cell–cell interaction/communication in type XII collagen–null bone in vivo. Analysis of cross sections from the diaphysis of femurs in P14 Col12a1^+/+ and Col12a1^−/− mice using transmission EM. (A) Wild-type osteoblasts (OB) attached to the surface of cortical bone (CB) are elongated and polarized relative to the bone matrix. In contrast, Col12a1^−/− osteoblasts (arrows) are disorganized and less polarized. PO, periosteum. (B and C) High magnification of adjacent osteoblasts in the periosteum. In wild-type bone, osteoblasts have numerous processes (double arrows) that connect osteoblast to osteoblast. Col12a1^−/− osteoblasts have large intracellular spaces (asterisks) with fewer processes (double arrows). (D) Osteocyte within cortical bone matrix. Similar to the osteoblast phenotype, an osteocyte has processes (double arrows) that extend toward the cortical bone surface in wild-type bone connecting to other osteocytes or osteoblasts; however, Col12a1^−/− osteocytes have fewer and less developed processes (double arrows). Bars: (A) 5 µm; (B) 1 µm; (C) 500 nm; (D) 2 µm.