Fig. 3. Timing of vertebral arch defect. (A,B,E,F,I,J) Safranin-O stained transverse sections through lumbar vertebrae at the indicated ages. (A,E,I) are control and (B,F,J) are PDGFR^PI3K/PI3K embryos. Safranin-O sections are representative of the furthest progression of the vertebral arch in the lumbar region at each time-point. By E15.5 the development of the vertebral arch in the mutant lags behind that of the control littermate, and failure of the vertebral arch to fully form was observed at E16.5. Arrowhead indicates condensing cartilage that fails to stain with Safranin-O. Black arrows denote furthest progression of vertebral arch. (C,D,G,H) H&E (in black and white) of (C,G) control and (D,H) PDGFR^PI3K/PI3K embryos at the indicated ages. (K,L) Hematoxylin and Eosin staining of E18.5 (K) control and (L) PDGFR^TKO. Asterisks indicate the lamina of the vertebral arch. White arrows indicate region of mesenchymal cells dorsal to the neural tube and their absence in the mutant embryos. Scale bars: 100 µm, except C,D,G,H (20 µm).