How to better build bone

Repairing damaged bones with tissue grafts is no easy task. Its success is limited by the challenge of regenerating the blood vessels that are needed to maintain the grafts. To meet this challenge, researchers at the University of Aveiro, in Portugal are developing layers of cell sheets that can support the formation of blood vessels in the process known as vascularisation.

The research team discuss the production of their “magnetic pre-vascularised cell sheets”, and their performance in laboratory trials, in the journal Biomaterials.

Making the cell sheets magnetic, by incorporating iron oxide nanoparticles, creates material that can be sculpted into 3-D shapes to match the needs for complex bone grafts.

“Despite notable advances in tissue engineering, repairing and regenerating bone is still a major struggle,” says Ana Sofia Silva, researcher at the COMPASS Research Group, from CICECO - Aveiro Institute. She explains that traditional and limited tissue engineering methods mainly use the injection of cell suspensions, or they use biodegradable scaffolds that support added cells. Both methods have significant limitations. For example, injected cells are deprived of the extracellular matrix that supports full bone development, while biodegradable scaffolds can be attacked by the host immune system and degrade at rates that are hard to control.

Creating sheets of cells embedded in their supporting extracellular matrix is emerging as a promising alternative. There are difficulties, however, in stimulating the all-important vascularisation process, in addition to the problems of controlling graft shapes.

The researchers made their magnetic cell sheets with a mixture of connective tissue cells derived from fat tissue and cells from umbilical cord tissue with the ability to promote the formation of new blood vessels. Manipulating individual sheets using magnets allowed them to be assembled into thick cellular multilayers that are more suitable for grafting.

In laboratory tests, the bio-molecular interactions between the cells stimulated the sculpted cell sheets to regenerate viable bone tissue. The detected effects included the release of vital growth factor molecules and the production of the proteins osteopontin and osteocalcin, which play crucial roles in bone regeneration. The cell sheets also promoted biomineralisation – the deposition of the calcium phosphate-based mineral hydroxyapatite – which brings crucial rigidity to bones.

The researchers then tested the activity of their cell sheets in chick embryos. The grafts successfully integrated into the developing chick tissue and recruited the new blood vessels needed to support graft survival and growth.

“This opens new prospects in the repair and regeneration of several bone disorders and fractures,” says Silva. She points out that the need for improved procedures is steadily increasing due the rising numbers of aged people who are more prone to fractures than the general population.

Further development and testing will be required before applying the new materials to specific bone repair tasks, but the early results are showing clear potential. Silva and her colleagues are also exploring ways to construct cell sheets with more complex structures and greater mechanical strength. She explains that this work includes gaining better control of the mixture and patterning of cell types to build more realistic tissues.

Article details:

Silva, A. S. et al.: “Multi-layer pre-vascularized magnetic cell sheets for bone regeneration,” Biomaterials (2020)