Using notochordal cells of developmental origin to stimulate nucleus pulposus cells and bone marrow stromal cells for intervertebral disc regeneration

Bone marrow stromal cells (BMSCs) have been proposed to complement the declining population of nucleus pulposus cells (NPCs) found in a degenerative intervertebral disc. Although able to stop degeneration, they could not produce enough matrix to restore a healthy state. Looking at development, when a large amount of matrix is produced, the disc also contains notochordal cells (NCs), potential progenitors or regulators of NPCs. The aim of the study was, therefore, to combine NCs to a BMSC/NPC mix and evaluate their effects on cell phenotype and matrix production, in long-term culture. In a 3D hydrogel, NCs were co-cultured in different ratios with BMSCs and/or NPCs. Matrix production, cell morphology, and gene expression of disc markers were assessed after 4 weeks of culture. At day 28, BMSCs/NPCs highly expressed disc matrix markers (type II collagen and aggrecan) and produced disc matrix up to 30 % of values obtained for the positive control (BMSCs under TGF beta stimulation). The addition of NCs only slightly up-regulated marker expression (6-12x increase); an up-regulation not reflected at the matrix level. During the 4 weeks of culture, however, the NC phenotype changed drastically (morphology, disc marker expression). In contrast to previously reported short-term studies, long-term co-cultures with NCs had no substantial effects on BMSCs and NPCs, most likely due to the loss of the NC native phenotype during culture. It, therefore, appears critical to maintain this specific phenotype for a long-term effect of the NCs.