Fig. 1. Induction of spina bifida by imatinib-mesylate. (A-F) Representative Safranin-O stained transverse sections through the lumbar and lumbar-sacral regions of chick embryos (HH stage 36). (A,D) Vehicle treated, and (B,E) 25 µM and (C,F) 50 µM imatinib-mesylate treated. The lamina (indicated by an arrow in A,D) failed to form across the dorsal midline in imatinib-treated embryos (indicated by arrowheads in B,C,E,F), resulting in spina bifida. Images show the most advanced arch formation found in serial sections of each treatment group. Scale bars: 200 µm. nt, neural tube; vb, vertebral body. Values listed below each column indicate the number of open vertebrae in all serial sections through the lower thoracic, lumbar and sacral region in each embryo. The number of embryos examined is indicated in parentheses. ^*Two out of four embryos treated with 50 µM imatinib-mesylate also exhibited failure of ventral closure.