Organogenesis MCB141 part III Spring 2014 Lecture 3. spinal cord Neural tube formation: 1. The neural ectoderm forms the neural tube during neurulation Gilbert6. Neurulation and somite formation progress from anterior to posterior, so that in one animal, several stages of somite formation and neurulation can be seen (Gilbert6). 2. Cell behaviors: Wedging (apical constriction), convergence and extension in the neural plate, medial migration of epidermis following from epiboly, fusion of the neural folds. These behaviors are particularly clear in the chick (Gilbert6) and the amphibian urodele embryo. Freeze fracture electron micrographs show the regions of bending in the floor plate and dorsolateral hinges. The urodele is represented in the figure from Gilbert6 and in this movie (1.6Mbytes). (by RMH) 3. Morphogenetic movements are summarized in this summary figure. The neural plate narrows, by the convergent extension of cells to the midline. This brings the folds closer together, so that the folds can reach each other and fuse, as shown in the diagram and the linked movies. The convergence and extension relies on cells being able to orient their behavior in th eplane of the tissue. This conserved process is the Planar cell Polarity (PCP) pathway, which uses upstream components of the Wnt signaling pathway, including Dishevelled, and diverges to use different proteins downstream, such as the Strabismus/VanGogh protein (Vangl in vertebrates). Mutations in the Vangl genes cause a failure of the neural tube to close in spina bifida. Even heterozygous mutations cause defects in some C/E processes, for example in the inner ear where there is a precise arrangement of sensory cells, and mutations in C/E components often cause deafness. 4. In addition to C/E bringing the neural folds together, the cells at hingepoints become wedge shaped, by apical constriction. The basis for apical constriction is not well-understood, but in the neural plate requires the expression of the shroom gene that mediates apical constriction in epithelial cells. When its function is blocked, neurulation fails, as shown in this paper (by Haigo et al)-see figure 5. 5. As the neural folds touch, they then fuse, and the neural tube segregates from the epidermis. Changes in cell adhesion contribute to the segregation of tissues: as mediated by cadherins (Gilbert6)