Organogenesis
At about the third week of prenatal development, the ectoderm of the embryonic disk forms the neural plate, which folds at the edges into the neural tube.
It begins to differentiate under the influence of the nearby notochord and paraxial mesoderm.
A group of cells migrates to form the neural crest, which gives rise to dorsal and autonomic ganglia, the adrenal medulla, and other structures.
The middle portion of the neural tube closes first; the openings at each end close later
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Schematic cross sections (A–F) showing the development of the spinal cord.
The cells in the wall of the neural tube divide and differentiate, forming an ependymal layer that encircles the central canal and is surrounded by intermediate (mantle) and marginal zones of primitive neurons and glial cells. The mantle zone differentiates into an alar plate, which contains mostly sensory neurons, and a basal plate, which is primarily composed of motor neurons. These two regions are demarcated by the sulcus limitans, a groove on the wall of the central canal. The alar plate differentiates into a dorsal gray column; the basal plate becomes a ventral gray column. The processes of the mantle zone and other cells are contained in the marginal zone, which becomes the white matter of the spinal cord.]
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Cross section showing two phases in the development of the spinal cord (each half shows one phase). A: Early phase. B: Later phase with central cavity.
An investing layer of ectodermal cells around the primitive cord forms the two inner meninges: the arachnoid and pia mater (pia) . The thicker outer investment, the dura mater (dura), is formed from mesenchyma.
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The spinal cord terminates at the L1 vertebral level in adults. Therefore, the lumbar and sacral nerve roots descending in the vertebral canal below the L1 vertebral level form a mass of nerve roots that resembles the tail of a horse; hence, the name cauda equina. Because the cauda equina floats in the CSF, a needle introduced into the subarachnoid space will displace the roots with little possibility of puncture damage.
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