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According to this

Neural Plate

The neural plate appears in day 18 in humans (very specific).

and according to this

Neural Plate (wiki)

The progenitor cells that make up the precursors to neural tissues in the neural plate are called neuroepithelial cells.

And according to this

Neuroepithelial cells

Neuroepithelial cells are the stem cells of the central nervous system, known as neural stem cells, and generate the intermediate progenitor cells known as radial glial cells, that differentiate into neurons and glia in the process of neurogenesis

So, if it's known when the cells which are precursors of neurons appear, I suppose it should be known when the first neurons appear, but I can't find information about it.

When do the first neurons appear in humans?

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Apparently, olfactory axons and GnRH producing neurons are among the first neurons that migrate from neural crest at around 39th day of gestation in humans (Cassoni et al., 2016).

In rodents, early GnRH neurons migrate together with a heterogeneous coalescence of placode-derived and neural crest-derived migratory cells (Forni et al., 2011) and olfactory axons, collectively called the ‘migratory mass’ (MM) (Miller et al., 2010; Valverde et al., 1992). This cell migration precedes the targeting of olfactory sensory axons to the developing olfactory bulb (OB). The existence of a similar MM in the human embryo has not yet been described. We thus immunolabeled consecutive sagittal sections of a CS 16 embryo for GnRH and doublecortin (DCX) (Fig. 1D,E,G), a marker of immature migratory neurons (Gleeson et al., 1999). We identified a very small number (50 in total) of immature GnRH-expressing cells in the nasal mesenchyme, in the medial portion of the olfactory placode (Fig. 1C-E), adjacent to the basal lamina of the VNO, showing that the acquisition of cell identity occurs outside the VNO between GW 5 and 6. At this stage, we observed a mixed mass of immature GnRH neurons expressing DCX (Fig. 1F,G) or βIII-tubulin (Fig. 1H-J), migrating across the nasal mesenchyme towards the telencephalon. As in rodents (Miller et al., 2010), GnRH neurons in humans only represented a small proportion of the MM (Fig. 1G). Furthermore, these pioneer neurons of the MM expressed the delta/notch-like EGF repeat containing (DNER) (Fig. 1H-J), a transmembrane protein specifically localized in the dendrites and cell bodies of postmitotic neurons.


Carnegie stage (CS) 16 = ∼39th day of gestation

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