Figure 5. Schematic representation of retinoid-dependent and -independent protocols for accelerated motor neurogenesis from hESCs.

SB431542 treatment accelerates neural induction and imposes a caudal positional identity. Subsequent treatment with a sonic hedgehog agonist (puromorphamine) and FGF2 (with or without RA) permits the generation of motor neurons. MN derivatives from the retinoid-independent pathway are biased to the caudal spinal cord (as depicted by the distribution and intensity of the red shading), the ventral aspect of the pMN domain and predominantly MMC fates (both medial and lateral subdivisions). MN derivatives from the retinoid-dependent pathway are biased to the rostral spinal cord (as depicted by the distribution and intensity of the red shading), the dorsal aspect of the pMN domain and promote the specification of LMC fates in addition to medial (but not lateral) MMC identities. The medial MMC emerges as a ground state for motor neurogenesis, being seemingly refractory to changes in retinoid signalling. Under retinoid-independent conditions, the lateral division of the MMC is also specified, while RA promotes the specification of LMC identities at the expense of lateral (but not medial) MMC MN specification. FGF2, fibroblast growth factor 2; L, lateral; LMC, lateral motor column; M, medial; MMC, median motor column; RA, retinoic acid; SB431542, an inhibitor of the activin/nodal signalling pathway.