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This review highlights the ontogenesis and the differentiation of motor neuron in spinal cord, and introduce the survival motor neuron(SMN) which is associated with growth and survival of motor neurons. The differentiation of floor plate cells and motor neurons in the vertebrate neural tube appears to be induced by signals from the notochord. This signal is Sonic hedgehog(Shh). The early development of motor neurons involves the inductive action of Shh. The SMN gene is essential for embryonic viability. SMN mRNA is also expressed in virtually all cell types in spinal cord, including large motor neurons. The SMN protein is involved in RNA processing and during early embryonic development is necessary for cell survival. Two SMN genes are present in 5g13 in- humans: the telomeric gene(SMNt), which is the SMA-determining gene, and the centromeric analog gene(SMNc). The majority of transcripts from the SMNt gene are full length but, major transcripts of the SMNc gene have a high degrees of alternative splicing and tend to have little or no exon 7. The SMN is involved in the RNA processing(the biogenesis of snRNPs and pre-mRNA splicing), the anti-apoptotic effects, and regulating gene expression.
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