DEVELOPMENT OF NEURONS

DEVELOPMENT OF NEURONS

12.      DEVELOPMENT OF NEURONS

The nervous system is derived from the ectoderm layer of the embryo. During initial development, the neuroectoderm appears and forms the neural plate on the dorsal side, which later on gives rise to all of the neurons and glial cells, and the phenomenon is termed as neurogenesis. The time of neurogenesis of a particular neuron is called as its birth date.

During the initial embryonic development, the neural plate is only a single layer of cells but at the closing of the neural tube, cells start to proliferate and layer thickened to increase the number of neurons because the neural plate is the major source of neuron and glial cells. The cells at the dorsal-most portion of the neural tube become the neural crest cells.

Ectodermal cells overlying the notochord are induced to form the neural plate in response to diffusible signals from mesoderm, called neural induction. The signal molecules like noggin and chordin are produced by dorsal mesoderm (notochord) and diffuse into the ectoderm to inhibit the BMP4 activity and to differentiate into neural cells. Neuronal differentiation is the process through which pluripotent neuronal stem cells adopt specific cell fates as different functional neurons.

The original neural tube is composed of a single layered germinal neuroepithelium of rapidly dividing neural stem cells (NSCs) dividing "vertically" instead of" horizontally." Thus, the cell adjacent to the lumen usually remains a neural stem cell and the other daughter neural progenitor cell (NPC) migrates away. These stem cells are continuous from the luminal surface the outside surface of the neural tube to, but their nuclei are at different heights because of movement regarding the cell cycle. During S phase of the cell cycle, the nucleus is an outer edge of the neural tube and then migrates luminally as the cell cycle proceeds.

Then the cells migrate to the distance proportional to their birth date and differentiate outside the neural tube. Neural stem cells are undifferentiated precursor cells divide to generate clonally related progeny that differentiate into neurons, astrocytes, oligodendrocytes, and ventricular ependymal cells. neural stem cells self-renewal and differentiation are regulated by a precise cascade of growth factors, intracellular signalling, and transcription factors (TF’s) expression as illustrated in the figure below.


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