The human intervertebral disc is an amazing structure that while at first appearance seems simple and static is actually quite the opposite. Each disc articulates with a vertebral body above and below it and provides three crucial services to the spinal column: 1) To support the axial load on the column as it is delivered by body mass and gravity 2) to assist in range of motion at the spine and 3) to serve ligamental functions between vertebral bodies.
The intervertebral disc is the largest avascular structure in the body. Performing its services as a lively, dynamic structure without a dedicated blood supply requires an efficient combination of form and function. It consists of three entities: the annulus pulposus (AP), nucleus pulposus (NP) and the vertebral end-plate (VEP). The AP surrounds the NP and forms the majority of the disc. These two parts are different in composition.
The AP is a fibrous structure composed of mostly collagen. It also contains some proteoglycan and elastins (most of which reside near the VEP). This collagen is arranged very specifically into approximately 20 concentric bands called lamellae. These collagenous rings align biomechanically to assist the NP in load bearing and also to resist the outward force of the NP as it receives the load.
The AP consists mainly of Type I collagen peripherally with an increase in Type II collagen as it gets closer to the NP. While these two structures are vastly different, their boundaries are difficult to establish grossly or microscopically. The NP has less collagen and more proteoglycan than the AP. Specifically, it uses a proteoglycan called aggrecan to trap water in the matrix. These aggrecan molecules are assembled onto long hyaluronic acid chains to produce larger aggregate molecules.
Aggrecan is a large negatively charged protein that has two functional moieties: a chondroitin sulfate and a keratan sulfate region. While these help resist compression, the protein's enormous negative charge creates an osmotic environment that attracts and retains water. The NP is 80% water due to this effect. The diurnal changes in load tendencies have an enormous impact on the water content in the NP. During the day, walking, standing and sitting upright creates an axial load on the spinal column that slowly pushes water out of the NP. When asleep, or resting prone/supine, the load is relieved which permits the negatively charged NP matrix time to re-attract water back into the matrix. This repetitive shift is crucial to the NP, as it permits vital nutrients and waste products to come and go, respectively, with the water content.
As a side note, with the diurnal movements of water into and out of the NP, it is accurate to say that a patient is tallest in morning, and will be shorter after a long day's work; estimates range between 1-3 cm total height difference depending on the height of the patient and the work!
The last component of the disc is the VEP. This is a cartilaginous plate that attaches the disc to the vertebral body above and below it. It is also the structure that participates in the diurnal water shifts that supply nutrients to the inner 2/3 of the AP and the entire NP.
At first glance, one would suspect the VEP to be apart of the vertebral body. However, embryologically the VEP is very much apart of outer AP lineage. Similarly, it should be noted that the NP and AP/VEP have distinctly different embryological development. The disc cells that inhabit the NP and AP even look different microscopically. This is in addition to their differences in function, as demonstrated by the differences in the aforementioned matrices they create.
While the NP and AP have little in common, they have an efficient, functional relationship. The relationship is symbiotic-?as the NP would squeeze out like toothpaste without the restraints placed upon it by the AP and the AP would waver without the NP holding it up aligned for axial load acceptance. A byproduct of this relationship between the two is an increase in disc pressure. This pressure, like that created by the air trapped inside a tire, permits axial load bearing and is a fundamental result of extremely efficient use of electrical charge, geometry and physics.
The human intervertebral disc is even more dynamic than described here. Disc cells are constantly making and taking down matrix components in response to variable stressors and genetic signaling. For more detailed information on the complex matrices within the intervertebral disc, as well as how it relates to disc degeneration and other pathology, a list of applicable resources include (not exhaustive by any means):
R Sztrolovics, M Alini, P J Roughley, and J S Mort. Aggrecan degradation in human intervertebral disc and articular cartilage. Biochem J. 1997 August 15; 326(Pt 1): 235-241.
Le Maitre CL, Pockert A, Buttle DJ, Freemont AJ, Hoyland JA. Matrix synthesis and degradation in human intervertebral disc degeneration. Biochem Soc Trans. 2007 Aug;35(Pt 4):652-5.
Le Maitre CL, Freemont AJ, Hoyland JA. Localization of degradative enzymes and their inhibitors in the degenerate human intervertebral disc. J Pathol. 2004 Sep;204(1):47-54.
Wenger KH, Woods JA, Holecek A, Eckstein EC, Robertson JT, Hasty KA. Matrix remodeling expression in anulus cells subjected to increased compressive load. Spine. 2005 May 15;30(10):1122-6.
Urban, Jill P.G., Roberts, Sally. The Intervertebral Disc?Normal, Aging, and Pathologic. Rothman-Simeone The Spine. 5th ed. Vol. 1, pp 71-79, 2006.
Marchand F, et al. (1990) Investigation of the laminate structure of lumbar disc anulus fibrosus. Spine. 1990; 15:402-410.
Raj, PP. (2008) Intervertebral disc: anatomy-physiology-pathophysiology-treatment. Pain Practice. 2008 Mar;8(1):18-44.
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