The Spine
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SECTION 2 OF 4:

Development

The striking segmented pattern of the human spine is established during embryogenesis, when the precursor of the vertebrae, the somites, are rhythmically added to the forming posterior part of the embryo. In humans, somite formation begins around the third week post-fertilization and continues until a total of around 52 somites are formed. The somites are epithelial spheres that contain the precursors of the vertebrae, the ribs, the skeletal muscles of the body wall and limbs, and the dermis of the back.

The periodicity (the tendency to recur at regular intervals) of somite distribution and production is thought to be imposed by a molecular oscillator, or clock, acting in cells of the presomitic mesoderm (PSM). Somites form soon after the beginning of gastrulation, on both sides of the neural tube, from a tissue called the presomitic mesoderm (PSM). The PSM is part of the paraxial mesoderm and is generated caudally (toward the tail) by gastrulation when cells ingress (enter) through the primitive streak and later through the tail bud.

Soon after their formation, somites become subdivided into the dermomyotome dorsally, which gives rise to the muscles and dermis, and the sclerotome ventrally, which will form the spine components. Sclerotomes become subvidided into an anterior and a posterior compartment. This subdivision plays a key role in the definitive patterning of vertebrae that form when the posterior part of one somite fuses to the anterior part of the consecutive somite during a process termed "resegmentation."  Disruption of the somitogenesis process in humans results in diseases such as congenital scoliosis.

So far, the human homologues of three genes associated to the mouse segmentation clock have been shown to be mutated in humans with human congenital scoliosis, suggesting that the mechanisms involved in vertebral segmentation are conserved across vertebrates. In humans, the first four somites are incorporated in the basi-occipital bone of the skull, and the next 33 somites will form the vertebrae. The remaining posterior somites degenerate.

During the fourth week of embryonic development, the sclerotomes shift their position to surround the spinal cord and the notochord. The sclerotome is made of mesoderm and originates from the ventromedial part of the somites. This column of tissue has a segmented appearance, with alternating areas of dense and less-dense areas.

As the sclerotome develops, it condenses further, eventually developing into the vertebral body. Development of the appropriate shapes of the vertebral bodies is regulated by HOX genes. The less-dense tissue that separates the sclerotome segments develop into the intervertebral discs.

The notochord disappears in the sclerotome (vertebral body) segments but persists in the region of the intervertebral discs as the nucleus pulposus. The nucleus pulposus and the fibers of the annulus fibrosus make up the intervertebral disc.