Cyclic Strain Versus Endothelial Cell Presence On MSC Osteogenesis

      Mesenchymal stem cells (MSCs) are increasingly recognized as a viable cell source for bone regeneration applications. In addition to soluble factors, cyclic stretch has been shown to have a profound effect on mesenchymal stem cell (MSC) osteogenesis. Similarly, endothelial cell (EC) presence has been demonstrated to enhance MSC osteogenic differentiation. However, potential synergistic interactions between mechanical stimulation and EC presence remain to be elucidated.  The aim of the present manuscript was therefore to examine the simultaneous influence of cyclic stretch and EC paracrine signaling on MSC osteogenesis in the context of scaffolds with “osteogenic” moduli.

            To accomplish this, 10T˝ multipotent stem cells were encapsulated in poly(ethylene glycol) diacrylate [PEGDA] hydrogels with moduli within the “osteogenic” range. Half of the constructs were fabricated with a luminal EC layer. EC⁺ and EC^- constructs were then subjected to continuous cyclic stretch. Following 10 days of culture, both EC⁺ and EC^- constructs were associated with significantly elevated levels of chondrogenic transcription factor sox9 relative to initial (day 0) expression levels. By day 22 of culture, however, sox9 levels in both the EC⁺ and EC^- constructs had returned to or fallen below day 0 levels. In contrast, osteocalcin expression was significantly higher in Day 22 EC⁺ constructs relative to EC^- constructs and relative to day 0. Similarly, osteopontin and alkaline phosphatase levels were elevated in Day 22 EC⁺ constructs relative to EC^- constructs. Cumulatively, the present results suggest that EC paracrine signaling enhances MSC osteogenesis in the presence of cyclic stretch. In addition, the observed transition from chondrogenic to osteogenic protein expression associated with the EC⁺ constructs would be consistent with the neovascularization-dependent transition from cartilage matrix to osteoid matrix associated with endochondral bone formation.