There is a shortage of donor tissue for grafting procedures to replace damaged human tissue. Tissue engineering holds great promise for generating tissue, but clinical success has been limited because the resulting tissue fails to integrate in the patient’s body after transplantation.
Tissue is complex, with multiple support structures, such as vascular and neural networks, containing different cell types. To engineer tissue for grafting purposes, this complexity must be replicated to ensure their correct functioning and integration. However, each tissue structure and cell type requires distinct environmental cues to develop properly.
The EU-funded PREVASCIN project addressed this important bottleneck in the field of tissue engineering by recreating the preferred local environment for developing different cell types. The researchers achieved this by creating a structural framework, described as “Living Lego”, combined with an elastic hydrogel matrix to which local growth factors can be added.
“The project used a building-block approach to combine different hydrogel formulations within single tissue constructs. In this way, local tissue development can be controlled, enabling the generation of complex tissues containing multiple structures,” says Jeroen Rouwkema, recipient of the project’s Marie Curie Action International Outgoing Fellowship, who was at Harvard Medical School in the USA and is now an associate professor at the University of Twente in the Netherlands.
“The project explored both the generation of bone tissue with neural structures and with endothelial structures,” he says. The ultimate aim is to engineer vascularised (with blood vessels) and innervated (with nerves) tissue from a single cell source.
The project developed a composite hydrogel system, comprising chemically modified gelatin (GelMA) and polyethylene glycol dimethacrylate (PEGDMA), with mechanical properties that can be fine-tuned by adjusting the percentage of PEGDMA. A type of human stem cell known as mesenchymal stromal cells (hMSC) – with the potential to differentiate into a variety of human tissues – preferentially transformed towards neural cells in soft gels and towards osteogenic cells (which make bone and bone marrow cell types) in stiff gels.
“We started from a single stem cell source to engineer an osteogenic tissue containing either a vascular or neural network, whereas previous studies combined cells from multiple sources,” explains Rouwkema. “By locally including the signals that result in the requested differentiation in the tissue building blocks, this can potentially simplify the procedure of generating complex multi-structural tissues.”
The project also developed a methodology to pattern growth factors inside the hydrogels, to support the specific organisation of bone tissue with endothelial cells. Therefore, the building blocks can be used to prepare tissue constructs containing different cell type regions determined by the hydrogel compositions. Thus, this approach provides for a high level of control over tissue development.
“The project has resulted in a clear advance in knowledge, and shown for the first time that an osteogenic tissue containing neural structures can be acquired from a single hMSC cell source by adapting local cellular environments,” says Rouwkema. “Development of the modular and self-assembly properties of the proposed approach has great potential to create a highly flexible system, easily translatable to other applications and engineered tissues.”
Further work is expected to improve the building- block approach for eventual use in clinical applications, which means the tools developed by the PREVASCIN project could have a highly significant impact in the very promising field of tissue engineering.