Nutrition of the intervertebral disk. An in vivo study of solute transport

The 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.