Changes in disk microstructure, composition, size, and vascular supply occur through- out growth and development.
Even at birth, the regions of the intervertebral disk are dis- tinct. The end plate has discrete hyaline cartilage that separates the vertebral body from the disk. The outer anulus fibrosus is com- posed of dense circumferential lamellae of collagen that penetrate the vertebral bodies of adjacent vertebrae. Small blood vessels may be found adjacent to the cartilaginous end plates; occasionally, blood vessels penetrate the inner anulus fibrosus. Numerous free nerve endings lie on and within the most peripheral layers of the anulus fibrosus.
During skeletal growth, disk volume and diameter increase several-fold, thus increasing the distance from the peripheral vessels and the central portions of the disk. Addition- ally, the three vessels of the end plate gradually disappear, leaving scars in the cartilaginous end plate, and the peripheral blood vessels of the anulus fibrosus become small- er and less numerous. As a result, the disk be- comes avascular. The concentration of cells de- crease as the nucleus pulposus becomes more dense and fibrous. By early adulthood, the em- bryonic cells have disappeared completely, leaving behind chondocyte like cells. The high water content of the nucleus pulposus also de- creases with age as the proportion of proteogly- cans that do not bind water progressively in- creases and the size of the aggrecan molecules decreases. This change in proteoglycans occurs in all regions of the disk but is greatest in the nucleus pulposus. Throughout all regions of the disk, the diameter and the variability of collagen fibrils increases.