Chondrocytes and fibroblastic cells are tissue constituents in the anulus, nucleus, and end plates. All these are are within an extensive, intricate extracellular matrix (ECM).
These cells are responsible for homeostasis of the ECM, including matrix formation, maintenance, and remodeling. The nutrition to these cells is derived from diffusion across the hyaline cartilage and subcortical end plates.
The cells in the central part of the disc lie closest to the end plate. By virtue of their position they lie nearest to vascular supply and are responsible primarily for nutrient and waste product exchange. The ultrastructure of the bony end plate comprises numerous sinusoids with specialized loops of capillaries that come into close contact with the underlying cartilaginous layer.
Simple diffusion of molecules across the cartilage occur depending on the size of small nutrients. As there is anaerobic metabolism due to anaerobic avascular nature of the disc, the latent pH within the disc under normal conditions is acidic.
The cells are responsible for production and constant remodeling of the principal components of the ECM, including collagen and proteoglycans (PG).
Collagen provides strength to the disc and is most abundant in the outer anulus; it makes up close to 70% of the dry weight of the anulus but only 20% of the dry weight of the nucleus. C
The concentric orientation of collagen fiber in the anulus is a series of sheets or lamellae of fibrils, each perpendicular to the adjacent layer. The densely packed layers consist principally of type I collagen, which is highly cross-linked for increased strength, and small amounts of types II, III, V, VI, and XI collagen.
Fibroblasts that diminish in number closer to the disc center are interspersed.
The inner anulus contains a relatively greater proportion of chondrocytes and varying amounts of type II collagen fibrils in a loose, nonorganized manner. The central nucleus is a gelatinous core containing chondrocyte-like cells and nearly 85% of type II collagen.
PGs consist of a protein core covalently attached to a glycosaminoglycan (GAG) subunit. The GAG units are hydrophilic in nature and retain water in its normal state. Normal water content within the disc is close to 70% to 90%.
Chondroitin sulfate and keratan sulfate are the most common glycosaminoglycans found in the disc, the former more prevalent in the normal. Multiple proteoglycan subunits are attach to a central hyaluronate filament by a link protein, a kind of glycoprotein.
The entire structure forms aggregate molecules, of which aggrecan is the largest found in the anulus, Others are versican, decorin, biglycan, and fibromodulin. Fibroblasts and chondrocytes are suspected within this mesh framework along with remnants of notochordal-like cells.
In the normal intervertebral disc, there is a gradient of the cellular and ECM components from the fibrous well-organized periphery to the randomly organized gelatinous center,
The gradient progresses though four less distinct zones of the disc
- Outer anulus
- Inner anulus
- Transition zone
- Central nucleus.
These compounds interact with water to provide the mechanical and structural integrity of the intervertebral disc.
The hydrophilic nature of the GAG molecules yields a highly hydrated amorphous central disc structure that in the normal state is constantly under expansible stress.
The nucleus retains water and expands, providing stiffness and resistance to compressive forces. This mechanism effectively transfers compressive load on the nucleus to a radially directed force with tensile stress to the anulus.
Constant load on the disc applies pressure to the nucleus, reducing the ability of the proteoglycans and, thus, the nucleus to hold water, causing creep of the disc.
After pressure is relieved, the time dependent viscoelastic properties of the disc nucleus allow it to regain water.
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About Dr Arun Pal Singh
