Neural induction: New insight into the default model and an extended four-step model in vertebrate embryos

Neural induction is a process by which naïve ectodermal cells differentiate into neural progenitor cells through the inhibition of BMP signaling, a condition typically considered the "default" state in vertebrate embryos. Studies in vertebrate embryos indicate that active FGF/MAPK signaling reduces BMP signaling to facilitate neural induction. Consequently, I propose that FGF stimulation/BMP inhibition more accurately characterizes the default model. Initially, the neuroectoderm is instructed to differentiate into anterior forebrain tissue, with cranial signals stabilizing this outcome. Subsequently, a gradient of caudalizing signals converts the neuroectodermal cells into posterior midbrain, hindbrain, and spinal cord. Furthermore, at the caudal end of the embryo, neuromesodermal progenitor cells are destined to differentiate into both neural progenitor cells and mesodermal cells, aiding in body extension. In light of these observations, I suggest incorporating an additional step, elongation, into the conventional three-step model of neural induction. This updated model encompasses activation, stabilization, transformation, and elongation.