In human and chick embryos, there appears to be a transitional region at the junction of the anterior (primary) and posterior (secondary) neural tubes. As mentioned earlier, neural tube formation in this transition zone is referred to as junctional neurulation (see Figure 13.4 in the textbook). In human embryos, coalescing cavities are seen in the transitional region, but the neural tube also forms by the bending of neural plate cells. The junctional neural tube in the chick is a mosaic of both ventral mesenchyme cells and dorsal neural ectodermal cells. In addition to directly providing dorsal epithelial cells to the junctional neural tube, neural plate cells also undergo an epithelial-to-mesenchymal transition and ingress into the underlying mesenchyme pool (Figure 1; Dady et al. 2014). Some posterior neural tube anomalies result when the two regions of the neural tube fail to coalesce (Saitsu et al. 2007). Given the prevalence of human posterior spinal cord malformations, further understanding of the mechanisms of secondary neurulation may have important clinical implications.
Dady, A., E. Havis, V. Escriou, M. Catala, and J. L. Duband. 2014. Junctional neurulation: a unique developmental program shaping a discrete region of the spinal cord highly susceptible to neural tube defects. J. Neurosci. 34: 13208–13221.