At the neural plate stage in the spinal cord region, the entire plate expresses the transcription factors PAX3, PAX7, MSX1, and MSX2, all of which contain a homeodomain. This expression pattern is altered by sonic hedgehog (SHH) expressed in the notochord and bone morphogenetic proteins 4 and 7 (BMP4 and BMP7) expressed in the nonneural ectoderm at the border of the neural plate (Fig. 19.14 A). The SHH signal represses expression of PAX3 and PAX7 and MSX1 and MSX2. Thus, SHH ventralizes the neural tube. This ventral region then acquires the capacity to form a floor plate, which also expresses SHH, and motor neurons in the basal plate. BMP4 and BMP7 expression maintains
and up-regulates PAX3 and PAX7 in the dorsal half of the neural tube, where sensory neurons in the alar plate will form (Fig. 19.145). These two genes are required for formation of neural crest cells in the top of the neural folds, but their roles and those of the MSX genes in differentiation of sensory neurons and interneurons is not clear. However, their expression throughout the neural plate at earlier stages is essential for formation of ventral cell types, despite the fact that their expression is excluded from ventral regions by SHH at later stages. Thus, they confer on ventral cell types competence to respond appropriately to SHH and other ventralizing signals. Yet another PAX gene, PAX6, is expressed throughout the elevating neural folds except in the midline, and this pattern is maintained after fold closure. However, the role of this gene has not been determined (Fig. 19.14 B).
Most defects of the spinal cord result from abnormal closure of the neural folds in the third and fourth weeks of development. The resulting abnormalities,
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