Figure 19.14 Molecular regulation of spinal cord development. A. Sonic hedgehog (SHH), secreted by the notochord, ventralizes the neural tube and induces a floor plate region (F) that also expresses this gene. Bone morphogenetic proteins 4 and 7 are secreted by the nonneural ectoderm and contribute to differentiation of the roof and alar plates. B. Initially, PAX3 and 7 and MSX I and 2 are expressed uniformly throughout the neural plate. SHH represses expression of these genes in the ventral half of the neural tube that will become the floor and basal plates. Simultaneously, BMPs upregulate and maintain expression of PAX 3 and 7 in the dorsal half of the neural tube that will form the roof and alar plates. PAX 6 begins expression throughout the neural ectoderm as the neural folds elevate and close. The exact roles of the PAX and MSX genes in differentiation of these regions have not been determined.

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