To understand the structure and function of the healthy, aging, and degenerative or traumatized intervertebral disc requires knowledge from the micro-level up to the macro-level, i.e., from subcellular components to the primary structures of the disc. It will help one understand the mechanism behind the normal, maturing or aging, and the injured or pathological disc. The field of cellular mechanobiology is still a young field as the cell’s substructures that had been speculated upon had only begun to be observed and described in the 1980s when better research tools became available. Much of the how, what, and whys are still unknown. What is known is that the structures, from molecules to major components, need to function flawlessly in order to maintain the health of the disc and the organism. They all require appropriate movement to maintain function. It is likely that the vertebral subluxation has a major deleterious effect on the chain of mechanisms involved, from subcellular to systems, and the physical stimulus of the adjustment in a positive direction will undoubtedly affect the structure of the disc, from subcellular to gross structures as well. This article in a series to update the Gonstead Disc Concept briefly explores the microscopic.
A Quick Natural History of the Intervertebral Disc—Macroscopic View
In childhood, the disc has the distinct features that we associate with and use in the model to describe the disc for patients. The anulus fibrosus has distinct lamellae with up to about twenty-five in the lumbar spine and only a handful in the cervical spine. The nucleus pulposus is glossy and gelatinous and distinct from the anulus. It has an abundance of water, as well as, negativelycharged glycosaminoglycans, collagens, and noncollagenous proteins (Setton & Chen 2004). The cartilaginous endplates are hyaline cartilage that extends to epiphyseal ring—Sharpey’s fibers anchor the anulus directly to the vertebral body lateral to the cartilaginous endplates. In fetal and early life, the endplates fill up much of the disc space. This state lasts for a short period of time.
True aging of the disc is probably not as common as discs with injuries and degenerative changes. This is probably the reason that it is difficult to develop a natural history of true disc aging and distinguish it from degeneration. Degeneration appears to be associated with changes to the extracellular matrix (ECM). Proteases, such as, matrix metalloproteinase (MMP), break down proteins in the ECM which results in the remodeling of the ECM. The proliferation of MMPs and other proteases occurs in the presence of cytokines and proinflammatory mediators. Although genetics is known to be a factor in the sequence that leads to degeneration, mechanical forces are thought to contribute as well (Setton & Chen 2004). Cells associated with degeneration begin to appear, such as, nerves, Schwann cells, endothelial cells, and fibroblasts. These cells are found with increased vascular supply (Setton & Chen 2004). The disc gets “messy” with injury and degeneration. The distinct anular lamellae of childhood develop fissure, , bifurcations, breaks, etc. which give the anulus an irregular appearance. The nucleus becomes fibrous and dehydrated, and the nucleus and inner anulus become indistinguishable. The endplates become thinner and often develop sclerotic changes that impairs nutrient transfer which causes further degenerative changes to the disc.
Although cells only make up 1% of the volume of the disc, their importance cannot be overstated. They produce the proteoglycans, type I and II collagen, MMP, prostaglandins, and nitro oxide that are factors in ECM turnover (Rannou et al 2004). Since the 1980s, there have been major discoveries about the cell’s substructures. Before, one learned that a cell had a semi-permeable membrane for nutrient and waste transfer, a cytoplasm and various other elements, and a nucleus with genetic material. Cell biologist have found that the cell membrane is incredibly complex and has a series of specialized “mechanoreceptors-like” molecules called integrins. These, in term, are connected to an internal framework in the cytoplasm called the cytoskeleton. Not only are integrins “receptors,” but they also attach the cell to the extracellular matrix and other cells and communicate with both, and help the cell move by popping in and out (wikipedia-integrin). The cytoskeleton works not only like a skeletal system but also tran smits external, mechanical forces from the extracellular matrix to the nucleus and back to the periphery: a mechanical nervous system.
The nucleus has its own “skeletal system,” the nucleoskeleton. Signals from the integrins via the cytoskeleton and external forces that deform the nucleus have an impact on the chromosomes and genes that can alter the function of the cell. Physical forces alter gene expression.
All of these subcellular structures are important in a concept borrowed from the late Buckmeister Fuller: the concept of tensegrity. The cytoskeleton and the other structures help to maintain the shape of the cell in spite of the twists and deformations that occur to them. The interlinked structures resist compression by maintaining isometric tension (Ingber 2010). Tensional forces are also created by cells adhering to other cells and to the ECM (Ingber 2010).
The cytoskeleton is formed by micro-tubules, microfilaments, and intermediate filaments. Microtubules resist compressive inward-directed forces which is balanced by tensile actomyosin filaments. Unlike are osseous skeletal system, the cytoskeleton is adaptable. If our osseous skeleton were as adaptable, we would be like the robot transformers in the cartoons. Another function of the cytoskeleton is that it works something like a powered tool rack. Many structures in the cytoplasm must attach to it for chemical actions to occur. There are specific catalytic actions that follow specific channels—a type of circulatory system? Some call this “solid-state” biochemistry as it uses physical connections to cause a biochemical reaction. In this way, there is mechanical forces being passed through the cytoskeleton and the subcellular structures attached to it have biochemical reactions. This is called mechanotransduction—the conversion of mechanical force to, in this case, biochemical response (Ingbar 2010).
Like the cytoskeleton, integrins multitask. They act as mechanoreceptors. They attach the cell to the ECM and other cells to allow bi-directional communications between the cell and the ECM and adjacent cells. Where cells attach to the ECM “scaffolding” or “focal adhesions” is where most of a cell’s integrins migrate to (Ingber 2010). At the intracellular end of integrins, they attach to actin-associated molecules, thereby linking the ECM to the cytoskeleton. In this way, integrins (and other transmembrane cell surface receptors) may function like mechanoreceptors for the cell (Ingber 2010). They pass mechanical forces to the cytoskeleton and nucleoskeleton and from the cell to the ECM and other cells that they are attached to. They act like feet for the cells. If the function of the integrins are interfered with, the interaction between the cell and external environment is altered and can lead to a pathophysiological state (Baker & Zaman 2010). Like the pathophysiological processes that result from a subluxation, the dysfunction of integrins can cause altered function. The cell’s ability to communicate is affected. The result of these sub-cellular interferences have a spreading effect on the organism.
The type of forces on the disc cells affect the shape of the cells. Towards the inner anulus where cells are sparse, the cells have a more rounded shape. Interlamellar cells have a more flattened or disc-shaped appearance. Anular cells with few attachments have a more rounded shape compared to those with extensive ECM or cell-to-cell attachments which are more flattened. Much of the cells of the anulus have an ellipsoidal shape with their long axis aligned with the lamellae (Setton & Chen 2004). This appears to correspond with Ingbar’s findings (Ingbar 2010). In other words, a bird has a certain shape and structure to fly while a bear does not need to be aerodynamic or light but needs to be muscular and bulky. Their structures take on characteristics for the lifestyle much like each cell takes on its particular shape and structural components for their functions.
The disc undergoes tensile and compressive loading, shear, osmotic and hydrostatic pressures, and vibrations. Each of these will have an influence on the cells of the disc, and the form will vary with the type and locations of the cells. The cell and its genes respond in a way that require specific cell structures that are appropriate for that region and the forces placed upon it. Examples include chondrocytes and endothelial cells which have nuclei that are significantly stiffer and more viscous than their cytoplasm compared to many other cells in order to resist deformation forces (Dahl et al 2010).
The fetal nucleus pulposus is populated by notochordal cells. After birth, these are rapidly removed, and by age 4 years, are mostly gone (Roberts 2002). The notochordal cells in the nucleus clump together in clusters and are so tightly bound that there is little extracellular fluid between the cells (Roberts 2002). The changes in forces that are exerted in the disc when the infant goes from being horizontal to sitting up to standing and walking probably is a key factor in the changeover along with the changes in the vascular supply. The literature that I looked at doesn’t explain this. Once the forces begin changing genetic expressions, cells that thrive in certain forces undergo mitosis and begin to proliferate. In the disc, these include chondrocytes but not notochordal cells. Many quadripedal animals retain notochordal cells. Maybe the weight-bearing forces upon the human discs probably require more robust cells like chondrocytes and other cell.
Setton and Chen state that mechanical stimuli causes remodeling in the disc (Setton & Chen 2004). Therefore, if motion is aberrant or the disc is unable to respond to normal motion, the disc must respond and change by adding fibrous-type cells and other types of cells. If the motion is altered beyond a certain range and/or for a sufficient length of time, the disc must remodel to adapt to the altered state. That would obviously lead to pathophysiological changes.
It would appear that a disc that has signs of injury, such as, damaged anular fibers, must adapt in order to stabilize the motion unit. If there is a loss of water and impaired synthesis of elements that form the disc matrix, the response of the disc is to move towards stability. Limiting movement is a means to this goal. This will require the need for fibrotic and supporting cells and further desiccation of the disc. One sees this in the Gonstead stages of disc degeneration.
Integrin: It is a protein on the cell’s surface that connects the cell to the ECM. They are called transmembrane heterodimers (proteins composed of two different macromolecules).
Cytoskeleton: This is the “skeletal system” of the eukaryotic cell (wikipedia-cytoskeleton).
Extracellular Matrix: That part of the tissues that provide support for the cells. One form is called interstial matrix. These fill the spaces along with polysaccharide gels and fibrous proteins between cells in animals. Another form of ECM is basement membrane. epithelial cells lie on these sheet-like structures. For injury repair and tissue engineering, the ECM has two main purposes: 1) stops the immune system from reacting to the injury and cause inflammation and scar formation, and 2) helps the tissues around the site of injury to repair rather than form scar tissue (wikipedia-extracellular matrix).
Lamins: These are intranuclear filaments that are composed of intermediate filament proteins that provide structure and regulate transcription in the cell nucleus. They form on the inner surface of the nuclear membrane and also are found throughout the nucleoplasm (wikipedia-lamin).
Nuclear Lamina: It is a filamentous structure in the nucleus which is connected to the inner nuclear membrane. It is also found elsewhere in the nucleus. It is formed of lamins. It is involved in nuclear functions, such as, DNA replication and transcription, organization of the nucleus and chromatins, cell cycle regulation, development and differentiation of cells (e.g., mitotic disassembly and reassembly of the nuclear envelope, nuclear migration, spacing of the nuclear pores, and apoptosis (Mattout-Drubezki & Gruenbaum 2003).
I hope that the subject matter wasn’t too esoteric. It is of vital importance in the health of the disc—and the entire body for that matter. It’s the foundation of all tissues. Movement is necessary to maintain each and every cell. The growth, shape, and genetic expression of almost all tissues and organs are influenced by mechanical forces (Wang et al 2009). How mechanotransduction, the process by which mechanical forces are changed or transduced by the cells to affect intracellular cytobiochemistry and gene expression is being studied. As pondered at the beginning, one wonders how mechanical forces altered by vertebral subluxation affects the cells of the disc, nerves, and other spinal structures that lead to the changes we find in the subluxated spine. One also wonders how the changes in mechanical forces following an adjustment affects the health and restoration of cellular function and growth of the spine and the effect on the rest of the body when there is restoration of disc and neural function.
Any feedback is welcome. As you can see, some of the conclusions are speculations on my part that were deduced from the literature.
Baker EL. Zaman MH. The Biomechanical Integrin. J Biomechanics 5 January 2010; 43(1):38-44.
Dahl KN, Booth-Gauthier EA, Ladoux B. In the Middle of It All: Mutual Mechanical Regulation Between the Nucleus and the Cytoskeleton. J Biomechanics 5 January 2010; 43(1):2-8.
Ingber DE. From Cellular Mechanotransduction to Biologically Inspired Engineering. Annals Biomedical Engineering March 2010; 38(3):1148-61.
Jammey PA, McCulloch CA. Cell Mechanics:Integrating Cell Responses to Mechanical Stimuli. Annals Biomedical Engineering 2007; 9:1-34.
Mattout-Drubezki A, Gruenbaum Y. Dynamic Interactions of Nuclear Lamina Proteins With Chromatin and Transcriptional Machinery. Cell Molecular Life Sciences October 2003; 60(10):2053-63.
Rannou F, Lee T, Zhou R, et al. Intervertebral Disc Degeneration: The Role of the Mitochondrial Pathway in Annulus Fibrosus Cell Apoptosis Induced by Overload. Am J Pathology March 2004; 164(3):915-24.
Roberts S. Disc Morphology in Health and Disease. Biochemical Society Transactions November 2002; 30(6):864-869.
Setton LA, Chen J. Cell Mechanics and Mechanobiology in the Intervertebral Disc. Spine 1 December 2004; 29(23):2710-23.
Wang N, Tytell JD, Ingber DE. Mechanotransduction at a Distance: Mechanically Coupling the Extracellular Matrix With the Nucleus. Nature Rev Molecular Cell Biology January 2009; 10(1):75-82.