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FIG 5. Schematic showing normal development of the intervertebral disk (adapted from Peacock [5]).
A, 15-mm embryo: mesenchymal cells that will form the intervertebral disks are arranged in parallel rows. The cells toward the center are more rounded and are irregularly arranged around the notochord. They have more intercellular substance between them and contain small capsules of cartilage, referred to as specialized embryonic cartilage.
B, 29-mm embryo: at the disk periphery, fibers of the annulus fibrosus are now present. These fibers run in parallel rows and cross the disk space; their terminations are lost in the adjacent vertebrae. More mature fibrocartilage is now present at the periphery of the specialized embryonic cartilage. Polygonal-shaped notochordal cells centrally are loosely arranged around an amorphous mucoid matrix.
C, Full term: bony trabeculae and intertrabecular marrow have developed within the vertebral body. No vestige of the intravertebral notochord is present. The notochordal region within the center of the intervertebral disk contains a faintly basophilic homogeneous mucoid substance. Small groups of residual notochordal cells can be found. At the circumference of the notochordal area, small groups of fibrocartilage are surrounded by the fibers of the annulus fibrosus.
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