Figure 2

EGFR signaling. Sections of the distal tibia of normal Mig-6-flox (A, C, E) and Mig-6-flox;Prx1Cre conditional knockout (Mig-6-cko) knees (B, D, F) immunostained with a phospho-EGFR antibody (p-EGFR, brown) and counterstained with hematoxylin. At postnatal Day 5 (A, B), p-EGFR signaling is occurring in the distal region of the tibial epiphysis (the presumptive articular cartilage) in both normal and Mig-6-cko knees, but staining is more robust in the Mig-6-cko knee and the domain of EGFR signal activation is expanded (compare bars in A, B). At six weeks of age (C, D), p-EGFR immunostaining in normal articular cartilage is limited to cells in the superficial zone (arrows in C), but in Mig-6-cko articular cartilage staining is intense and abundantly localized in both superficial and middle regions (arrows and arrowheads, respectively, in D). At 12 weeks of age (E, F), p-EGFR-positive chondrocytes are only occasionally observed in the superficial and calcified zones of normal articular cartilage (arrows and arrowheads in E); whereas p-EGFR-positive chondrocytes are present in superficial, middle and calcified zones of the Mig-6-cko articular cartilage (arrows and arrowheads in F). Scale Bar = 50 μm (A-F).