Chapter 13: Neural Tube Formation & Patterning

The process begins with the specification of the ectoderm, where varying levels of bone morphogenetic proteins (BMPs) determine the fate of the tissue: high levels lead to the formation of the epidermis, low levels induce the neural plate, and intermediate levels generate the migratory neural crest. The physical construction of the neural tube occurs through primary and secondary neurulation. In primary neurulation, the neural plate elongates and folds inward at specific hinge points, a process driven by apical constriction and cell shape changes that allow the tissue to curve and eventually fuse at the midline. This folding is precisely regulated by a molecular interplay where morphogens such as Sonic hedgehog (Shh) and Noggin antagonize BMP signaling to permit tissue bending at the correct locations. As the neural folds meet, differential expression of adhesion molecules like N-cadherin and E-cadherin ensures the tube separates successfully from the overlying skin. Any disruption in these closure events can result in severe congenital anomalies, such as spina bifida or anencephaly, which are linked to both genetic factors and environmental influences like folate deficiency. Once the tube is formed, it undergoes regionalization along its axes. The anterior portion expands into primary vesicles that will become the forebrain, midbrain, and hindbrain, while the posterior matures through secondary neurulation involving mesenchymal condensation. Patterning along the dorsal-ventral axis is achieved through opposing gradients of Shh from the ventral notochord and TGF-beta family signals from the dorsal epidermis. Neural progenitor cells interpret these signals based on both the concentration and the duration of their exposure, utilizing Gli transcription factors and complex gene regulatory networks characterized by mutual cross-repression to establish sharp boundaries between different neuronal cell types. Finally, the rostral-caudal axis is matured through the integration of Fgf, Wnt, and retinoic acid gradients, ensuring that the entire nervous system is properly wired and localized for future development.