An understanding of developmental biology can provide useful insights into how different tissue-engineered repairs might be designed. During embryogenesis of the intervertebral disk, the cells of the notochord play a critical role in initiating tissue formation, and may be responsible for development of the nucleus pulposus. In some species, including humans, these notochordal cells may eventually be lost, either through apoptosis or terminal differentiation, and are replaced by chondrocyte-like cells. However, there is some evidence that the notochordal cells may persist in at least some humans. This review discusses some of the potential applications of notochordal cells in tissue engineering of the nucleus pulposus.

The nuclei pulposi of the intervertebral discs (IVDs) contain a mixed population of cell types at various stages of maturation. This tissue is formed either by or with the help of cells from the embryonic notochord, which appear to be replaced during development by a population of chondrocyte-like cells of uncertain origin. However, this transition occurs at widely varying times, depending upon the species -or even breed -of the animal being examined. There is considerable debate among spine researchers as to whether the presence of these residual notochordal cells has a significant impact upon IVD degeneration models, and thus which models may best represent the human condition. The present study examines several different species commonly used in lumbar spine investigations to explore the variability of notochordal cells in the IVD.

The three-dimensional architecture of cells in the annulus fibrosus was studied by a systematic, histological examination using antibodies to cytoskeletal components, in conjunction with confocal microscopy. Variations in cell shape, arrangement of cellular processes and cytoskeletal architecture were found both within and between the defined zones of the outer and inner annulus. The morphology of three, novel annulus fibrosus cells is described: extended cordlike cells that form an interconnected network at the periphery of the disc; cells with extensive, sinuous processes in the inner region of the annulus fibrosus; and cells with broad, branching processes specific to the interlamellar septae of the outer annulus. The complex, yet seemingly deliberate arrangement of various cell shapes and their processes suggests multiple functional roles. Regional variations in the organization of the actin and vimentin cytoskeletal networks is reported across all regions of the annulus. Most notable is the continuous, strand arrangement of the actin label at the disc's periphery in contrast to its punctate appearance in all other regions. The gap junction protein connexin 43 was found within cells from all regions of the annulus, including those which did not form physical connections with surrounding cells. These observations of the cellular matrix in the healthy intervertebral disc should contribute to a better understanding of site-specific changes in tissue architecture, biochemistry and mechanical properties during degeneration, injury and healing.

Cells from the nucleus pulposus of young (< 2 years) and old (> 5 years) non-chondrodystrophoid dogs were studied using routine histology, confocal laser scanning microscopy and transmission electron microscopy. The architecture of cell structures -from the tissue scale down to subcellular scale -was reported. Clusters of notochordal cells were observed in young nuclei pulposi, ranging from 10 to 426 cells each. These clusters resisted mechanical disruption and showed evidence of cell-cell signalling via gap junctions. Cells (30-40 µ m in diameter) within the clusters had a physaliferous appearance, containing numerous large inclusions which ranged from 1 to 20 µ m in diameter. The inclusions were surrounded by a dense actin cortex but were not contained by a lipid bilayer. The contents of the inclusions were determined not to be predominantly carbohydrate or neutral lipid as assessed by histochemical staining, but the exact composition of the contents remained uncertain. There were striking differences in the cell architecture of young vs. old nuclei pulposi, with a loss of both cell clusters and physaliferous cells during ageing.These observations demonstrate unique cell structures, which may influence our understanding of the differences between notochordal and chondrocytic cells in the nucleus pulposus. Such differences could have substantial impact upon how we think about development, degeneration and repair of the intervertebral disc.

Objective. To study 3 body fluids for changes in the levels of 5 biomarkers of cartilage metabolism during the early phases of experimental osteoarthritis (OA).Methods. Twenty skeletally mature mixed-breed canines underwent unilateral surgical transection of the anterior cruciate ligament. Samples of joint fluid, serum, and urine were obtained preoperatively and just before necropsy (3 weeks or 12 weeks postoperatively). Biomarkers included 2 markers of cartilage matrix synthesis/turnover (aggrecan 846 epitope and C-propeptide of type II collagen) and 3 markers of cartilage degradation (keratan sulfate proteoglycan epitope, the collagenase-generated cleavage epitope of type II collagen [Col2-3/4C long mono , or CIIC], and crosslinked peptides from the C-telopeptide domain of type II collagen [Col2CTx]). Significant changes in the levels of these biomarkers were determined by paired analyses.Results. Joint pathology was more severe in the 12-week group compared with the 3-week group. In joint fluid, due to limited volume, only Col2-3/4C long mono and Col2CTx were measured. Significant elevations in the levels of both of these markers were observed in experimental joints in both the 3-week group and the 12-week group. In serum, the level of aggrecan 846 epitope was elevated at both 3 weeks and 12 weeks, the level of Col2-3/4C long mono was elevated at 12 weeks, and the level of Col2CTx was elevated at both 3 weeks and 12 weeks. In urine, the level of Col2-3/4C long mono was elevated at 12 weeks after surgery.Conclusion. Levels of biomarkers of intact aggrecan proteoglycan (aggrecan 846 epitope) and type II collagen degradation (Col2-3/4C long mono and Col2CTx) were elevated early after unilateral stifle joint injury, suggesting that these markers are sensitive and specific for early cartilage changes associated with isolated joint injury in this established model of experimental OA.The early stages of osteoarthritis (OA) are difficult to diagnose. Joint structure and function are typically altered substantially before symptoms cause patients to seek medical care, that is, the osteoarthritic process begins long before OA presents as a clinical disease. The insidious onset and "silent" progression of primary OA not only obscure an early diagnosis, but also delay treatment that may help prevent further cartilage destruction and joint failure. Hence, diagnostic tests that can detect and monitor molecular events early in the pathogenesis of OA would be potentially very useful.For nearly a century, synovial fluid analysis has been important in the differential diagnosis of arthritis. Although synovial fluid analysis is useful in distinguishing between inflammatory, septic, and crystal-induced arthritis, its value in OA is limited. Similarly, routine blood and urine analyses can identify certain inflammatory arthritides and metabolic defects that cause joint pain and pathology but, as yet, reveal little about the OA process. No specific molecular markers of the OA process are currently available for clinical use, and the...

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Objectives: To detect changes in the collagen fibril network in articular cartilage in a canine experimental model of early osteoarthritis (OA) using microscopic magnetic resonance imaging (mMRI) and polarised light microscopy (PLM). Methods: Eighteen specimens from three pairs of the medial tibia of an anterior cruciate ligament transection canine model were subjected to mMRI and PLM study 12 weeks after surgery. For each specimen, the following experiments were carried out: (a) two dimensional mMRI images of T 2 relaxation at four orientations; (b) the tangent Young's modulus; and (c) two dimensional PLM images of optical retardance and fibril angle. Disease induced changes in tissue were examined across the depth of the cartilage at a mMRI resolution of 13.7-23.1 mm. Results: Several distinct changes from T 2 weighted images of cartilage in OA tibia were seen. For the specimens that were covered at least in part by the meniscus, the significant changes in mMRI included a clear shift in the depth of maximum T 2 (21-36%), a decrease in the superficial zone thickness (37-38%), and an increase in cartilage total thickness (15-27%). These mMRI changes varied topographically in the tibia surface because they were not significant in completely exposed locations in medial tibia. The mMRI results were confirmed by the PLM measurements and correlated well with the mechanical measurements. Conclusion: Both mMRI and PLM can detect quantitatively changes in collagen fibre architecture in early OA and resolve topographical variations in cartilage microstructure of canine tibia.