Figure 1.

Developmental sites of origin of OLPs. The diagrams depict the sites of origin and migration pathways of OLPs during embryonic and perinatal development. (a) Drawings of coronal sections through the developing telencephalon at the indicated ages. Colour marks the sites of origin of OLPs in the VZ and arrows show their presumed migration pathways. (b) Transverse sections of the cervical spinal cord showing the early ventral and later dorsal sources along with their dispersal routes. Note: The brain sections are drawn to scale relative to each other, as are the spinal cord sections, but the spinal cord is drawn disproportionately large compared with the brain. The ventral sources of OLPs are induced by Shh from the floor plate of the spinal cord or the VZ of the ventral forebrain. The dorsal sources (including the cerebral cortex of the brain) are thought to be Shh independent, possibly relying on fibroblast growth factor. Dorsal OLP generation might also be helped by BMP downregulation in dorsal structures during later embryonic development. See text for a more complete description. E13, embryonic day 13; E15, embryonic day 15; PO, postnatal day 0.