Diversity of intervertebral disc cells: phenotype and function

Abstract

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.

Fig. 1

Schematic representation of an intervertebral disc (IVD). The cartilaginous endplate (CEP) and the adjacent vertebrae (VB) are visualized in the sagittal section of the disc (A). The annulus fibrosus (AF) is formed by structured lamellae which encapsulate the central nucleus pulposus (NP) (B).

Fig. 2

Gross morphology of a bovine caudal IVD from a 6-month-old calf, from different viewpoints. The concentric rings of the annulus fibrosus and central gelatinous core of the nucleus pulposus can be visualized in the transversal section of the IVD (A). The shape of the disc and the calcified portion of the endplates can be appreciated from a lateral view of the disc (B). Blood vessels are present in the outer part of the cartilaginous endplate, as observed in the top view of the IVD (C).

Fig. 3

Safranin O-Fast-green staining of a caudal IVD from a 6-month-old bovine calf showing the distribution of sulphated glycosaminoglycans and collagen across the disc. An outer AF region rich in collagen and a central NP region rich in glycosaminoglycans are observed in the overview of the disc (A). The AF is formed by concentric lamellae (B) and its cells follow the orientation of these lamellae (E). The transition zone between AF and NP is characterized by a gradual decrease in collagen, increase in glycosaminoglycans and loss of the lamellar structure (C); cells of the AF are generally elongated (F). The NP matrix is mainly composed of glycosaminoglycans (D) and cells of the NP generally have a rounded morphology (G). Scale bars: 500 μm (A), 200 μm (B–D), 20 μm (E–G).

Fig. 4

Immunostaining for cytokeratin-19 (A–C) and carbonic anhydrase 12 (CA12) (D–F) in human nucleus pulposus tissue. Notochordal-like cells of a 3-year-old individual stain positive for both markers (A and D). Nucleus pulposus cells of a 14-year-old individual also show immunoreactivity for both markers (B and E), whereas positive staining is hardly observed in normal adult IVD (not shown). However, pathological tissue of degenerated discs (C: 79 years and F: 62 years) can react positive for cytokeratin-19 and CA12. Scale bar: 100 μm. Original references are from Rutges et al. (2010) and Power et al. (2011).

Fig. 5

Relative mRNA expression for carbonic anhydrase 12 (CA12) in nucleus pulposus cells of seven individuals between 25 and 81 years of age. Gene expression was normalised to four housekeeping genes. A decrease in the CA12 expression with (A) increasing age and (B) disc degeneration level (Thompson grade) is noted. Original reference is from Power et al. (2011).