Fig. 5. OLPs arise from the intermediate and dorsal regions of the neuroepithelium after dorsal spinal cord ablation. The dorsalmost region of the spinal cord in E3 embryos was excised in ovo or, in control experiments, was simply opened without removing dorsal tissue. In both cases, this led to the formation of a spina bifida. Operated embryos were fixed at E7-7.5. Transverse sections were stained with O4 antibody. (A) Left hemisection from a control spinal bifida embryo (dorsal is towards the top). The spinal cord is widely opened and the ventricular surface is exposed dorsally. O4 antibody delineates a ventricular focus of OLPs of normal extension (bracket), close to the floor plate (fp). (B) Right hemisection from a dorsal spinal cord excision. The ventricular focus of O4-positive cells (bracket) is enlarged compared with that found in the control embryo. (C) Section in another embryo with dorsal spinal cord excision. The position of the sulcus limitans is denoted by the broken line on the left side. Ectopic O4-positive cells (arrows) are found away from the original focus of oligodendrogenesis (bracket), evenly spaced along the neuroepithelium up to the sulcus limitans. (D-F) Section in an embryo with dorsal spinal cord excision, at a level where the spinal cord remained closed. (D) The broken lines indicate the level of the sulcus limitans. Note the important development of O4-positive cells in the marginal zone. (E,F) Enlargements of the areas boxed in D. Ectopic O4-positive cells (arrowheads) develop dorsally to the sulcus limitans (E) and in the intermediate neuroepithelium (F). Note one O4-positive cell that seems to have migrated away from the dorsal neuroepithelium (arrow in E).

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