Fig. 4. mCHL2 mRNA expression in cartilage. In situ hybridization (ISH) for CHL2 in normal mouse embryos, depicted in paired bright-field (left) and dark-field (right) panels. (A) Expression of CHL2 mRNA at the costochondral junction, at E17.5. Note CHL2 in chondrocytes on both sides of the junction. (B) Transverse section through sternum, at E18.5. Signal is present in areas where ribs converge. (C,D) ISH for CHL2 in the adult knee. Weak CHL2 expression is present over the articular cartilage surface of the femoral head and patella, but not in growth plate chondrocytes. Boxed areas in C shown at a higher magnification in D in which the signal can be seen in chondrocytes on both sides of the joint. (E,F) Expression of CHL2 in adult vertebral articulation. Signal occurs in superficial articular chondrocytes on both sides of the zygapophyseal or facet joint. The boxed area in E is shown at a higher magnification in F, revealing signal localization over the superficial zone chondrocytes. dv, dorsal (superior) vertebra; vv, ventral (inferior) vertebra. Arrowheads indicate CHL2-positive chondrocytes.