FIGURE 1.

The development of SOC in the proximal epiphysis of the tibia is delayed in the chondrocyte-specific EGFR-deficient mice. A, immunohistochemistry of EGFR in the epiphyseal cartilage of P9 WT mice reveals that most chondrocytes express EGFR (a). Black arrows point to EGFR negative periarticular region. Negative control (b) shows immunostaining with omission of the primary antibody. B, primary epiphyseal chondrocytes derived from Egfr CKO mice exhibit decreased ERK phosphorylation (p-ERK) in response to EGF when compared with those from WT and Wa5 mice as shown by immunoblots. Cells were harvested at the indicated times after 50 ng/ml EGF treatment. C, safranin O-stained sections from WT, Wa5, and Egfr CKO mice at the age of 5 (a–c), 9 (d–i), 12 (j–l), and 18 (p–r) days show the SOC and growth plate development. Double-headed arrows indicate the hypertrophic zone. Cartilage canals and hypertrophic chondrocytes in the SOC are indicated by yellow arrowheads and yellow arrows, respectively. g–i, magnified images from d–f, respectively. m–o, darkfield images of in situ hybridization with probes to col2a1 at the SOC in mice at the age of 12 days. D, consecutive sections from methyl methacrylate-embedded samples, one stained with Goldner's trichrome (a–c and g–l) and the other stained with von Kossa (d–f and m–o), reveal matrix calcification in the hypertrophic cartilage (black arrows) in the SOC from mice at the age of 9 (a–f) and 12 (g–o) days. j–l, magnified images from g–i, respectively. The yellow arrowhead in i points to a cartilage canal.