Abstract
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.
The abstracts were prepared by Orah Naor. Correspondence should be addressed to him at the Israel Orthopaedic Association, PO Box 7845, Haifa 31074, Israel.
