Figure 1

Runx2 and VDR are co-expressed in the osteoblastic lineage in amniotes but not in amphibians. The figure shows longitudinal sections of hindlimbs from E7 Gallus gallus (A-B, E-L) or from NF58 Xenopus tropicalis (C-D) specimens. (A) Hematoxilin staining showing the preosteoblasts of the developing perichondrium surrounded by hypertrophic chondrocytes, connective tissue and skeletal muscles. (B) Localization of the VDR protein in the skeletal muscles and cells of the osteoblastic lineage. (C) Hematoxilin staining showing the osteoblasts of the developing periosteum surrounded by hypertrophic chondrocytes and skeletal muscles. (D) Localization of the VDR protein in the skeletal muscles. (E) Hematoxilin staining of the perichondrium. (F-H) Fluorescent immunohistochemistry showing the localization of VDR (F), Runx2 (G) and the colocalization of both proteins (H) in the perichondrium. (I-L) Higher magnification of the perichondrium cells. Nuclei are visualized using ToPro either alone (I, blue channel), with VDR (J, green channel), Runx2 (K, red channel), or both proteins (L). Abbreviations: Ch, hypertrophic chondrocytes; Ob, cells of the osteoblastic lineage; Ct, connective tissue; Sm, skeletal muscles. Scale bars represent 25 micrometers in a-d; and 10 micrometers in e-h and i-l.