Publication date: 25th July 2016
Skin is a complex organ composed of heterogeneous cell populations precisely arranged throughout the epidermal and dermal layers. Fibroblasts are the predominant cells in dermis, responsible for the production of components of the connective tissue that maintain tissue integrity. As the skin as limited self-regenerative properties after injury, cell-laden hydrogels have been widely explored to promote skin repair, owing their ability to recapitulate key properties of native extracellular matrix (ECM).In this study, we report the biofunctionalization of photocrosslinkable pectin hydrogels aimed to be used as provisional cellular matrices for the repair of dermal wounds. Pectin was rendered photocrosslinkable by the introduction of methacrylates through the reaction with methacrylic anhydride. Cell-laden hydrogels were obtained through the radical-initiated chain-growth polymerization of methacrylated pectin (PectMA) macromers (1.5% wt) loaded with human neonatal dermal fibroblasts (hNDFs, 1x107 cells/ml) in the presence of irgacure 2959 (0.05%wt) and UV light (7 mW/cm2, 160s). After photopolymerization, hydrogels were incubated in cell culture medium containing calcium chloride (1.8 mM), yielding a secondary physical gelation between carboxylic groups in PectMA and calcium ions in culture medium. Rheological analysis of acellular hydrogels immediately after photocrosslinking revealed an elastic modulus (G’) of 79.6±7.7 Pa, which significantly increased to 2548.9±132.1 Pa after overnight incubation in culture medium. To elucidate cell-ECM interactions and decouple the effects of peptide ligands on cell behaviour, hNDFs were embedded in hydrogels functionalized with either cell-adhesive peptide (CGGGGRGDSP) or plasmin-sensitive peptide (GCYK↓NRCGYK↓NRCG), and combination of both. Cells were viable for the duration of the study (14 days) in all hydrogel compositions demonstrating their cytocompatibility. Cell morphology and ECM deposition were highly influenced by both peptide ligand type and its concentration. Our results showed that in hydrogels without peptide functionalization, cells were able to maintain high levels of viability, but remained mostly round and isolated. Cell-adhesive peptides were essential for cell attachment and spreading within the hydrogels, though it was not enough to promote full cell functionality as ECM components (fibronectin and collagen) were only detected in the vicinity of the cells. In hydrogels containing high concentration of plasmin-sensitive peptide, cells were not only able to spread and attach, but also formed a fibronectin/collagen mesh throughout the matrix. Furthermore, cells organized in 3D spheroids within the gel matrix. When embedded in hydrogels functionalized with both peptides, cells were uniformly distributed throughout the hydrogel forming an intricate ECM network.