Patterning of The Nervous System
In chordates, dorsal ectoderm forms all neural tissue and the nervous system. Patterning occurs due to specific environmental conditions - different concentrations of signaling molecules
Dorsoventral axis
The ventral half of the neural plate is controlled by the notochord, which acts as the 'organiser'. The dorsal half is controlled by the ectoderm plate which flanks the neural plate on either side.
Ectoderm follows a default pathway to become neural tissue. Evidence for this comes from single, cultured cells of ectoderm which go on to form neural tissue. This is postulated to be because of a lack of BMPs, which are blocked by the organiser. The organiser may produce molecules such as follistatin, noggin and chordin which inhibit BMPs.
The ventral neural tube is patterned by Sonic Hedgehog (Shh) from the notochord, which acts as the inducing tissue. Notochord-derived Shh signals to the floor plate, and induces Shh expression in the floor plate. Floor plate-derived Shh subsequently signals to other cells in the neural tube, and is essential for proper specification of ventral neuron progenitor domains. Loss of Shh from the notochord and/or floor plate prevents proper specification of these progenitor domains. Shh binds Patched1, relieving Patched-mediated inhibition of Smoothened, leading to activation of Gli family of transcription factors (Gli1, Gli2, and Gli3) transcription factors.
In this context Shh acts as a morphogen - it induces cell differentiation dependent on its concentration. At low concentrations it forms ventral interneurones, at higher concentrations it induces motor neuron development, and at highest concentrations it induces floor plate differentiation. Failure of Shh-modulated differentiation causes holoprosencephaly.
The dorsal neural tube is patterned by BMPs from the epidermal ectoderm flanking the neural plate. These induce sensory interneurones by activating Sr/Thr kinases and altering SMAD transcription factor levels.
Rostrocaudal (Anteroposterior) axis
Signals that control anteroposterior neural development include FGF and retinoic acid which act in the hindbrain and spinal cord. The hindbrain, for example, is patterned by Hox genes, which are expressed in overlapping domains along the anteroposterior axis under the control of retinoic acid. The 3' genes in the Hox cluster are induced by retinoic acid in the hindbrain, whereas the 5' Hox genes are not induced by retinoic acid and are expressed more posteriorly in the spinal cord. Hoxb-1 is expressed in rhombomere 4 and gives rise to the facial nerve. Without this Hoxb-1 expression, a nerve which is similar to the trigeminal nerve arises.
Read more about this topic: Neural Development
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