Cartilaginous tissues such as articular cartilage and the intervertebral disc are called upon to function under very high pressures which they can do, thanks to the very special properties of their two major components, viz., the proteoglycans (PG) and collagen. The PG, a flexible polyelectrolyte of high fixed charge density has a high osmotic pressure and therefore a tendency to imbibe water and maintain tissue turgor while the collagen mesh, with its good tensile properties, prevents undue swelling, thus enabling the proteoglycan-water mixture to exist as a concentrated solution. Moreover, by resisting instantaneous deformation, the collagen network ensures the dimensional stability of cartilage. The combination of the two components enables a cartilaginous tissue to exhibit flexibility and to withstand tensile stresses as well as high compressive loads.

Moreover, cartilage is an avascular tissue, hence the transport of nutrients and different substrates is controlled by the properties of the matrix. In addition to common nutrients, various regulatory substances, such as growth hormones and cytokines, also have to reach the cell. These substances are often required in extremely small amounts which, however, need to be rigorously controlled. This again, depends on transport through the extracellular space. At the same time, metabolic waste products are secreted by the cells into the matrix and have to pass through the latter in order to reach the synovial fluid for removal from the joint space. The same must happen to matrix macromolecules degraded in the course of normal turnover, whether the degradation happens intra- or extracellularly. Finally, macromolecules, newly synthesized by the cells, are secreted into the matrix and must move through it before being assembled at some distance from the cell.

The concentration of a solute within the matrix, apart from being an important factor in determining the rate of transport, is also able to modify the properties of the matrix itself. Thus, ionic concentrations are largely responsible for determining the level of the osmotic pressure within the cartilage matrix in general, and in the immediate environment of the cell in particular. The osmotic pressure of the matrix, in turn, is responsible for the resistance of cartilage to fluid loss and hence to compressive stresses. Together with the hydraulic permeability of the pore space, it is also an important determinant of the rate of fluid movement out of and into the tissue. In addition, the high ionic concentration and osmotic pressure in the immediate environment of the chondrocyte have been shown to affect their synthetic processes.

1. Serial slices of articular cartilage obtained at necropsy from apparently normal femoral condyles of individuals between the ages of twenty-six and sixty were examined chemically, by electron microscopy and for permeability.

2. The most superficial layer was shown by chemical analysis and electron microscopy to have the highest collagen content, which fell sharply with distance from the articular surface. On the other hand the glycosaminoglycan content was very low in the superficial layers but increased with depth. This variation was found in all specimens tested but the absolute levels of collagen and of glycosaminoglycans were widely different. There was no correlation of chemical composition with age.

3. Collagen fibrils in the superficial layer were of much smaller diameter than in the deeper zones.

4. Hydraulic permeability was shown to depend more on glycosaminoglycan than on collagen content, although it varied inversely with both these factors.

5. The results obtained demonstrate clearly the close relation between the physical properties of cartilage and its chemical composition.

1. We have shown that the permeability of cartilage is the same in necropsy specimens as in the living animal. We have concluded that studies of material transport into cartilage carried out on necropsy specimens validly reflect in vivo conditions.

2. We have studied the effect of agitation of the fluid in which cartilage is immersed upon the rate of diffusion of substances into cartilage and have found that agitation increases the rate of penetration up to three or four fold. We believe that it may be inferred from this fact that the nutrition of cartilage is partly dependent on joint movement.

3. We have examined the permeability of the bone-cartilage interface to water and solutes and have found that in the adult no detectable material transfer occurs across this zone. In the child on the other hand the bone-cartilage interface appears to be permeable to water and solutes.

4. We have measured the diffusion coefficient of glucose in cartilage and have hence estimated the depth of cartilage which can be adequately supplied with glucose from the synovial fluid in the presence and absence of agitation.

5. We have examined both experimentally and theoretically the possible effect of intermittent loading on the rate of penetration of substances into cartilage. We have concluded that at low pressures intermittent loading contributes little to the material transfer into cartilage. At high pressures intermittent loading does lead to the transport of solutes into cartilage but it cannot significantly increase the rate of transfer above that attributable to normal diffusion. Loading cartilage surfaces for prolonged periods of time without allowing intermittent relaxation would be expected to lead to decreased diffusion, without any absorption of fresh fluid attributable to the action of a pump, and would thus result in an overall decrease in the rate of penetration of substances into cartilage.