Assessment of the cellular heterogeneity of the ovine intervertebral disc: comparison with synovial fibroblasts and articular chondrocytes

Abstract. The objective of this study was to compare and contrast the cellular heterogeneity of ovine intervertebral disc (IVD) annulus fibrosus (AF) and nucleus pulposus (NP) cells, synovial fibroblasts (SFs) and articular chondrocytes (ACs) grown in monolayer and alginate bead culture. Phase contrast microscopy of the monolayers indicated that the SF and AF cells, and the AC and NP cells had similar morphologies. Immunolocalisation of type I, II and VI collagen epitopes in the monolayers, however, demonstrated distinct quantitative and relative differences between the SF and AF, and the AC and NP cells. Immunolocalisation of bead-incorporated proteoglycans (PGs) also demonstrated quantitative and qualitative differences between the SF and AF cells. Quantitation of 35S-bead PGs indicated that the AF cells synthesised significantly higher PG levels than the SFs, and AC cells synthesised significantly higher PG levels than the NP cells. These data were also consistent with the relative metabolic activities and cellular viabilities displayed by each cell type in bead culture. AF cells shared morphological traits with SFs in monolayer, but displayed distinctly different characteristics in bead culture. Similarly, NP cells shared similarities with ACs in monolayer and bead culture, but overall were less metabolically active. Data presented here support the proposal that the AF and NP cells of the ovine IVD should be considered as specialised fibrochondrocytic cells.