Spinal thought 208. The first notion of what is to become the spinal cord and the spine is one of the earliest developments in the embryo. Already at a couple of weeks gestation there appears something called a notochord in the endoderm, ectoderm and mesoderm layers of cells in the raspberry shaped embryo called the blastocyst. The notochord is a flexible rod-shaped body composed of cells derived from the mesoderm and this defines the primitive axis of the embryo. Eventually it will become the nucleus pulposus of the intervertebral disc. It is found in front of the neural tube. Next, at about 3 weeks gestation, a striking segmented pattern is established when somites are added behind the embryo. These somites are formed by the paraxial mesoderm on the sides of the neural tube and contain the precursors of the spinal bone including skull, vertebrae and ribs as well as muscles and skin. In us humans the first 4 somites make up the occipital bone and the next 33 will for the vertebrae. The embryological growth gallops ahead and already a week later a development called sclerotomes, originating from the somites, shift position and surround the spinal cord and notochord. This tissue is segmented and as it continues to develop will condense further to develop into the vertebral body. I recall my teacher Don Burton describing the notochord in the layers of the embryo as a peanut butter sandwich with a celery stalk in the middle. (Photo from D. Gorman, The Body Moveable.)
Posted on: Sun, 27 Jul 2014 10:35:26 +0000
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