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We studied the mechanical and biochemical properties of articular cartilage from 22 osteoarthritic femoral heads obtained at operation and 97 femoral heads obtained at autopsy. Cartilage from the zenith and from the antero-inferior aspect of each head was tested both in tension and in compression. Water content, swelling ability and proteoglycan content were measured, the cartilage was examined histologically and the density of the underlying bone was assessed. Fifty-five of the autopsy specimens were defined as macroscopically normal because they exhibited no progressive fibrillation patterns on staining with Indian ink; but significant changes in water content, bone density and tensile strength related to age were seen in this group. In 20 pairs of femoral heads which were both macroscopically normal, we found, surprisingly, that cartilage from the left and right sides of the same patient was sometimes very different. Compared with the normal autopsy specimens the osteoarthritic specimens had a significantly increased swelling ability, a lower proteoglycan content and impaired mechanical properties, being both weaker in tension and softer in compression. Abnormal autopsy specimens had values intermediate between those of osteoarthritic and normal groups. Results from this abnormal group suggest that there is no primary loss of proteoglycan in early osteoarthritis.
\n \n\n \n \nIn this study we have considered how equilibrium water content of the human nucleus pulposus varies with applied pressure for discs of various spinal levels and of various ages. In all cases hydration decreased as pressure increased but the level of equilibrium hydration depended on the relative amounts of collagen and PG in the tissue. Provided we accounted for the exclusion of PGs from the intra-fibrillar space, the swelling pressure curve and the osmotic pressure curve of equivalent PGs were found to coincide. The result implies that under physiological hydrations the mechanical forces exerted by the collagen network of the nucleus are insignificant and that the osmotic pressure of the proteoglycans is balanced by the applied pressure arising from body weight and muscle and ligament tension alone. Since aged discs often have a low proteoglycan to collagen ratio, their equilibrium hydration also tends to be low. Moreover a far larger proportion of the total water is associated with the collagen than in the younger disc.
\n \n\n \n \nAdult dogs were injected intravenously with 35S-sulphate, and moderately exercised for one to six hours to measure isotope concentrations and profiles throughout the intervertebral discs. The isotope profiles were also observed in control animals that had been under anesthesia between injections and death. In both sets of animals, the profiles were in agreement with those expected for isotope transport by diffusion. This agreement indicates that fluid \"pumping\" during movement has an insignificant effect on transport of nutrients into the disc. Small solutes, e.g., O2, glucose, and sulphate, are transported into the disc chiefly by diffusion. However, calculations show that because of their low diffusivities, \"pumping\" may increase the rate of transport of large solutes into the disc, as it does in articular cartilage.
\n \n\n \n \nSlices of the intervertebral disc swell rapidly in aqueous solutions. Following swelling, a large proportion of the proteoglycans leach out. These changes can introduce artifacts into in vitro experimental work. Swelling results from the large Gibbs-Donnan osmotic pressure of the proteoglycans which the collagen network of the disc is unable to oppose alone. Swelling can be prevented by applying pressure to disc slices either mechanically or with iso-osmotic solutions.
\n \n\n \n \nThe author has shown in this chapter that whilst at the moment there is still a considerable shortage of experimental data relating to the chemical and physical properties of the intervertebral disc, the broad physicochemical principles underlying its physiology are now fairly clear. It particular, it should become possible in the future to describe the nutrition of the intervertebral disc and its mechanical behavior in terms of the basic structure of the tissue and the well defined physicochemical properties of its components.
\n \n\n \n \nProfiles of fixed charge density across young and old disc were measured by a modification of the tracer-cation method which prevented proteoglycan loss. The fixed charge density of the nucleus was 0.28 mequiv./g in the young disc and 0.18-0.24 mequiv./g in the old disc. The fixed charge density varied steeply across tha annulus, falling to about 0.07-0.13 mequiv./g in the outer annulus. Profiles of the partition coefficients for small ionic solutes were calculated using these values of fixed charge density, measured profiles of water content and the Gibbs-Donnan equilibrium equations. Measured values of the sulphate ion partition coefficient compared well with the calculated values. \u00a9 1979.
\n \n\n \n \nThe main mechanism for solute transport within the intervertebral disk is passive diffusion. The 2 routes for the exchange of solutes with the blood vessels outside the disk are via the periphery of the annulus, and through the end-plates. While the periphery of the annulus is completely permeable, the bone--disk interface is only partially so. In the region of the nucleus the effective area through which solute transport is taking place constitutes some 85% of the actual bone/disk interface; in the region of the inner annulus it is reduced to only 35% while the bone--disk interface at the outer annulus is almost completely impermeable. These figures, calculated from tracer diffusion experiments correlate very well with the qualitative observations of blood vessel contact. Apart from its dependence on the permeability of the endplate, solute diffusion is also determined by the nature of the solute. For example, a negatively charged solute such as the sulphate ion is considerably excluded from the nucleus, which limits its rate of penetration via the endplates. The sulphate uptake by the disk cells to produce glycosaminoglycans is low and comparable to that in articular cartilage.
\n \n\n \n \nPost-mortem specimens of the human lumbar (L4-L5) intervertebral disc have been studied histologically and physico-chemically. Blood vessels were found only at the margin of the anulus fibrosus and in the vertebral marrow spaces. Contact between disc tissue and marrow spaces occupied about 10% of the bone-cartilage interface. The disc was most cellular at the periphery of the anulus fibrosus and in the hyaline cartilage next to the vertebral bone. Cellularity declined towards the nucleus pulposus where it achieved a low constant value. The cell density of the disc as a whole was about 60000 cells/mm3. For glucose, the diffusion coefficient of the anulus fibrosus and hyaline cartilage end plate was 2.5 cm2/sec and 2.4 cm2/sec respectively at 37 degrees C, comparable to that of cartilage elsewhere. The permeability of the bone-cartilage interface was low, particularly in the peripheral part. Calculations, based on the present findings and derived values for glucose utilization in disc tissue, indicate that nutritional conditions in the intervertebral disc are more critical than, for example, in articular cartilage.
\n \n\n \n \nPost mortem specimens of the human lumbar (L4 L5) intervertebral disc have been studied histologically and physico chemically. Blood vessels were found only at the margin of the anulus fibrosus and in the vertebral marrow spaces. Contact between disc tissue and marrow spaces occupied about 10% of the bone cartilage interface. The disc was most cellular at the periphery of the anulus fibrosus and in the hyaline cartilage next to the vertebral bone. Cellularity declined towards the nucleus pulposus where it achieved a low constant value. The cell density of the disc as a whole was about 6000 cells/mm3. For glucose, the diffusion coefficient of the anulus fibrosus and hyaline cartilage end plate was 2.5 cm2/sec and 2.4 cm2/sec respectively at 37\u00b0C, comparable to that of cartilage elsewhere. The permeability of the bone cartilage interface was low, particularly in the peripheral part. Calculations, based on the present findings and derived values for glucose utilization in disc tissue, indicate that nutritional conditions in the intervertebral disc are more critical than, for example, in articular cartilage.
\n \n\n \n \nThe cells of the intervertebral disc exist in an unusual environment. They are embedded in a dense matrix containing a high concentration of aggrecan whose fixed negative charges regulate the extracellular ionic composition and osmolarity; both extracellular cation concentrations and osmolarity are considerably higher than those experienced by most cell types. The disc also is avascular. Oxygen levels in the centre of the nucleus, where cells may be 6-8 mm from the blood supply, are very low. Since metabolism is mainly by glycolysis, lactic acid is produced at high rates and hence the pH is acidic. Finally, the disc is subjected to mechanical forces at all times; these vary with posture and activity. In particular, because the disc is under low loads during rest and high loads during the day's activities, it loses and regains around 25% of its fluid over a diurnal cycle with consequent changes to the concentrations of extracellular matrix macromolecules and ions and hence extracellular osmolality. Here we will briefly review these factors and discuss the influence of changes in the physicochemical environment on cellular activity, in particular on the rate at which disc cells synthesize and degrade matrix macromolecules.
\n \n\n \n \nThe intervertebral disc is the largest asvacular structure in the body; consequently, there are steep gradients in O2 concentration, with PO2 falling to as low as 1% O2 in the centre of the disc. We investigated the effect of O2 concentration on the rates of O2 consumption, lactate production, and sulphate and proline incorporation in bovine caudal discs. We also investigated the effects of metabolic inhibitors of energy production pathways on tracer incorporation. Samples from the outer annulus and nucleus pulposus were incubated for 24 hours in 1-21% O2. Rates were measured during the last 4 hours of incubation. As O2 concentration was reduced from 10 to 1% O2, O2 consumption rates fell by around 75% and lactate production rates almost doubled; the bovine discs thus showed a positive progressive Pasteur effect. Incorporation rates of [3H]proline and [35S]sulphate were lowest at 1% O2. In the nucleus, but not in the outer annulus, the rate of [35S]incorporation peaked at 5% O2, where it was 30% greater than at 21% O2 and 150% greater than at 1% O2. The competitive glycolysis inhibitor 2-deoxyglucose, the oxidative phosphorylation uncoupler 2,4-dinitrophenol, and the oxidative phosphorylation inhibitor sodium azide all markedly reduced sulphate incorporation. These results, together with previous measurements of CO2 production rates, suggest that a functionally significant fraction of the disc's energy is supplied by oxidative phosphorylation. However, low levels of PO2, 2,4-dinitrophenol, and sodium azide have been reported to reduce sulphate incorporation in articular cartilage, a tissue that derives its energy almost entirely from glycolysis.
\n \n\n \n \nThe charge structure of the surface of articular cartilage determines its interactions with the macromolecules and cells of synovial fluid. It may thereby be important to the physiological function and pathological degeneration of the tissue. To determine whether the electrokinetic properties of the surface differ from those of the bulk tissue, we measured the streaming potential generated by the flow of electrolyte over the surface of a cartilage plug mounted in a chamber built for that purpose. We then calculated the effective surface charge density. In nonfibrillated cartilage from the human femoral head, the surface charge density, 0.037+/-0.004 Cm(-2) (mean+/-SD), was approximately half that measured at the surface of slices cut from the middle and deep zones. In addition, the surface charge density fell relatively little at low pH; this is consistent with a higher proportion of strongly acidic groups. The variations in surface charge density were found to be similar to those in total fixed charge density in the slices by the tracer cation method. Therefore, no evidence exists that the actual surface differs in composition from the immediately underlying matrix. The addition of synovial fluid (0.0025 ml/ml) to the superfusing solution reduced the surface charge density by 25+/-9% (n=5), and we attributed this to the binding of synovial-fluid macromolecules.
\n \n\n \n \nThe intervertebral discs undergo profound changes in structure and composition during development and aging. They also degenerate much earlier than other cartilaginous tissues, and in severe cases may lose function completely. The reasons for this early degeneration are unknown, but external factors, such as mechanical overload on the spine, or smoking, may be involved. However, recent work has revived interest in the importance of genetic and developmental influences on the intervertebral discs.
\n \n\n \n \nMechanical load is an important regulator of chondrocyte metabolic activity. Changes in amplitude or frequency of the load can have a significant effect on the production of matrix macromolecules and of agents leading to cartilage breakdown. The composition of cartilage reflects the net response of the chondrocytes to the prevailing loading pattern, with cartilage proteoglycan content highest in heavily loaded regions and removal of load leading to cartilage thinning and proteoglycan loss. The mechanism of mechanotransduction is poorly understood; the chondrocytes appear to react to cartilage deformation and to the changes in hydrostatic pressure, extracellular ionic composition and streaming potentials induced by the load.
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