Specification of a neuron

Birth of a neuron involves several sequential steps orchestrated by signaling events.4 The initial step is the specification of neuroepithelia from the embryonic ectoderm — a process known as neural induction. This step takes place during the third week of human embryonic development. By the end of the third week, a sheet of columnar neuroepithelia, referred to as the neural plate, has formed and has begun to fold and form the neural tube. The mechanism of neuroectoderm specification, inferred largely from studies using xenopus, chick, and other low vertebrates, remains a topic of scientific debate. The main division is whether neural induction occurs simply by removal of inhibitory factors, such as bone morphogenetic proteins (BMPs), the so-called "default pathway," or requires active inductive signaling such as by fibroblast growth factors (FGFs).5 It is likely that both instructive and inhibitory signalings are necessary. FGF may instruct a "pro" neural state at an early stage, and BMP antagonists may subsequently stabilize the neural identity.

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