T h e nucleus pulposus is a key player in very early disc degeneration. In the young disc, by acting as a water-like fluid, as opposed to a solid, it resists compression and instantaneously distributes forces evenly in all directions to the inner annulus. T h e disc anlage notochordal cells contribute not only to how the disc develops, but also to the matrix of the young disc at a time when the nucleus is at its most fluid-like. In humans, the notochordal cells disappear early, when there is a transformation of the nucleus into a more solid cartilaginous tissue. In cell culture, the co-cultures of the notochordal cells and chondrocytic cells enhance proteoglycan synthesis by the opposite cell type due, at least partly, to soluble factors. T h e continued presence of notochordal cells in vivo may provide protection. In work by others, in vivo reinsertion of notochordal-rich nucleus pulposus in a damaged disc will delay annular degeneration. T h e notochordal cells in the nucleus may have a different phenotype from when they are in the notochord and they may go through a changing programme of expression critical to disc development and maintaining a fluid-like nucleus. Little is known about why, in many species, the notochordal cells die early during growth and only the chondrocytic cells persist. This area offers an interesting avenue of research that may lead to very early intervention in disc degeneration. ~ ). imaging in the nucleus pulposus [3] soon after the disappearance of notochordal cells [4].Current opinion is that the presence of notochordal cells modifies the chondrocyte-like nucleus pulposus cells and is required to maintain the ideal nucleus pulposus extracellular matrix. This ideal matrix is an incompressible, fluid-like, well-hydrated proteoglycan (PG) gel in a loose
Proteoglycans are major components of many extracellular matrices. In cartilage, they provide reversible resistance to compression and exist as molecules with molecular weights (MWs) of 1-3 x 10(6). There is a central protein core of MWs approximately 2 x 10(5) (refs 1, 2) with specialized subregions, one containing mainly the chondroitin sulphate chains, another most of the keratan sulphate chains, and a third is a largely globular structure interacting specifically with both hyaluronic acid and a link protein to form stable aggregate structures such as those identified in human articular cartilage. In embryonic and tissue culture systems, proteoglycans are isolated as aggregate structures in as little as 5--10 min after synthesis (sulphation) with no nonaggregating precursor detected. However, Heinegärd and Hascall have characterized the small proportion of nonaggregating proteoglycan present in bovine nasal septum cartilage and found that it contained more peptide than the aggregating proteoglycan. Work by Upholt et al. has suggested that the MW of unprocessed protein core, synthesized by a wheat-germ translating system from chick sternal cartilage mRNA, is approximately 340,000, leaving open the possibility of intermediates. I report here the presence, in some human cartilages, of a proteoglycan population that initially will not aggregate with the hyaluronic acid but subsequently can be chased into aggregate.
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