Spinal Dysraphism

cells migrate between the layers of ectoderm and endoderm to form the embryonic mesoderm. Hensen's node is located at the cranial end of the primitive streak and, from here, the notochordal process develops in a cranial direction between the two embryonic layers by day 17. The solid notochord becomes a hollow cylindrical structure, which transiently fuses with the underlying layer of endoderm. There exists, therefore, a communication between the amni-otic cavity dorsally and the yolk sac ventrally via the primitive pit, known as the neurenteric canal. This communication closes as the noto-chord again separates from the endoderm by day 20. At this time, Hensen's node and the primitive streak regress with growth of the embryonic disc, in a caudal direction, ultimately to lie in the low sacral or coccygeal region.

The notochord induces the overlying ectoderm to thicken and cells heap up to form the neural plate. The neural groove develops in the neural plate, producing lateral folds which ultimately meet in the midline as the neural groove deepens, to form the neural tube. This is the process of primary neurulation. Closure of the neural tube begins in the mid-thoracic region and extends both cranially and caudally. At the cranial end of the neural tube (the future lamina terminalis), closure of the anterior neuropore occurs by day 24, while closure of the caudal or posterior neuropore occurs by day 28. The location of the posterior neuropore is a matter of some debate but probably lies in the region of L1 or L2. Caudal to this level, development of the spinal cord does not occur by primary neurulation. In this region, Hensen's node and the primitive streak give rise to an undifferenti-ated clump of cells known as the caudal cell mass, destined to form the conus medullaris and the filum terminale. This occurs by a rather poorly defined process - secondary neurulation - of vacuolation, condensation and subsequent fusion to the spinal cord formed by primary neurulation. The significance of this process in the human remains rather uncertain.

As the process of primary neurulation occurs, the ectoderm lateral to the developing neural plate fuses in the midline to cover the neural tube, while embryonic mesoderm from the scle-rotome at each level of the embryo migrates towards the midline to surround the notochord and the neural tube to give rise ultimately to the vertebral bodies and neural arches, as well as the dural sheath. The process of secondary neurulation (vacuolation, condensation and fusion of the caudal cell mass) occurs after the overlying ectoderm has fused to form the skin.

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