The intervertebral disc (IVD) is a moderately moving joint that is located between the bony vertebrae and provides flexibility and load transmission throughout the spinal column. The disc is composed of different but interrelated tissues, including the central highly hydrated nucleus pulposus (NP), the surrounding elastic and fibrous annulus fibrosus (AF), and the cartilaginous endplate (CEP), which provides the connection to the vertebral bodies. Each of these tissues has a different function and consists of a specific matrix structure that is maintained by a cell population with distinct phenotype. Although the healthy IVD is able to balance the slow matrix turnover of synthesis and degradation, this balance is often disturbed, leading to degenerative disorders. Successful therapeutic management of IVD degeneration requires a profound understanding of the cellular and molecular characteristics of the functional IVD. Hence, the phenotype of IVD cells has been of significant interest from multiple perspectives, including development, growth, remodelling, degeneration and repair. One major challenge that complicates our understanding of the disc cells is that both the cellular phenotype and the extracellular matrix strongly depend on disc maturity and health and as a consequence are continuously evolving. This review delineates the diversity of the cell types found in the intervertebral disc, with emphasis on human, but with reference to other species. The cells of the NP appear rounded and express a proteoglycan‐rich matrix, whereas the more elongated AF cells are embedded in a collagen fibre matrix and the CEPs represent a layer of cartilage. Even though all disc cells have often been referred to as ‘intervertebral disc chondrocytes’, distinct phenotypical differences in comparison with articular chondrocytes exist and have been reported recently. The availability of more specific markers has also improved our understanding of progenitor cell differentiation towards an IVD cell phenotype. Ultimately, new cell‐ and tissue‐engineering approaches to regenerative therapies will only be successful if the specific characteristics of the individual tissues and their context in the function of the whole organ, are taken into consideration.

Human mesenchymal stem cells (MSCs) reside under hypoxic conditions in vivo, between 4% and 7% oxygen. Differentiation of MSCs under hypoxic conditions results in inhibited osteogenesis, while chondrogenesis is unaffected. The reasons for these results may be associated with the inherent metabolism of the cells. The present investigation measured the oxygen consumption, glucose consumption and lactate production of MSCs during proliferation and subsequent differentiation towards the osteogenic and chondrogenic lineages. MSCs expanded under normoxia had an oxygen consumption rate of ∼98 fmol/cell/h, 75% of which was azide-sensitive, suggesting that these cells derive a significant proportion of ATP from oxidative phosphorylation in addition to glycolysis. By contrast, MSCs differentiated towards the chondrogenic lineage using pellet culture had significantly reduced oxygen consumption after 24 h in culture, falling to ∼12 fmol/cell/h after 21 days, indicating a shift towards a predominantly glycolytic metabolism. By comparison, MSCs retained an oxygen consumption rate of ∼98 fmol/cell/h over 21 days of osteogenic culture conditions, indicating that these cells had a more oxidative energy metabolism than the chondrogenic cultures. In conclusion, osteogenic and chondrogenic MSC cultures appear to adopt the balance of oxidative phosphorylation and glycolysis reported for the respective mature cell phenotypes. The addition of TGF-β to chondrogenic pellet cultures significantly enhanced glycosaminoglycan accumulation, but caused no significant effect on cellular oxygen consumption. Thus, the differences between the energy metabolism of chondrogenic and osteogenic cultures may be associated with the culture conditions and not necessarily their respective differentiation.

Mesenchymal stem cells (MSCs) are an attractive cell source for tissue engineering applications due to their multipotentiality and increased expansion potential compared to mature cells. However, the full potential of MSCs for cellular therapies is not realised, due, in part, to premature proliferative senescence and impaired differentiation capacity following expansion under 20% oxygen. Bone marrow MSCs reside under reduced oxygen levels (4%-7% oxygen), thus this study investigates the effects of uninterrupted physiological oxygen tensions (2%, 5%) on MSC expansion and subsequent differentiation. Expansion potential was evaluated from colony formation efficiency, population-doubling rates, and cellular senescence. Colony formation was significantly reduced under 5% oxygen compared to 2% and 20% oxygen. Population-doubling time was initially shorter with 20% oxygen, but subsequently no significant differences in doubling time were detected between the oxygen conditions. MSCs expanded with 20% oxygen contained a greater proportion of senescent cells than those under physiological oxygen levels, indicated by a three to fourfold increase in β-galactosidase staining. This may be related to the approximately twofold enhanced mitochondrial oxygen consumption under this culture condition. Chondrogenic differentiation was achieved following expansion at each oxygen condition. However, osteogenesis was only achieved for cells expanded and differentiated at 20% oxygen, indicated by alkaline phosphatase activity and alizarin red staining. These studies demonstrate that uninterrupted hypoxia may enhance long-term MSC expansion, but results in a population with impaired osteogenic differentiation potential. Thus, novel differentiation conditions are required to enable differentiation to nonchondrogenic lineages using hypoxia-cultured MSCs.

We have demonstrated for the first time that degenerative conditions induce the release of factors promoting BMSC recruitment in an ex vivo organ culture. Moreover, IGF-1 transduction of BMSCs strongly increases the rate of proteoglycan synthesis within degenerative discs. This finding offers a new delivery system for BMSCs and treatment strategy for IVD regeneration.