We’ve heard about 3D bioprinting, 3D cell culture and 3D tissue engineered constructs. Creation of three-dimensional tissue constructs in the last decade allowed to advance regenerative medicine tremendously! The most sophisticated 3D stem cell culture could be turned into organ-like structures. Well, we recently ditch 2D, but in the last few years we also realize that 3D is still not a limit. What if we add one more dimension, which allows cells in suspension to self-organize autonomously? Self-assembly is a fourth dimension of tissue engineering.
What is self-assembly?
Self-assembly could be defined as “autonomous organization of components, from an initial state into a final pattern or structure without external intervention”. One of good examples of human-made 4D structures is a printing combined with self-assembly. Importantly, self-assembly is a universal property of many living organisms and non living things on Earth. What amazes me the most that self-assembly is nothing more than harnessing nature’s power to self-organize. One of the best examples of self-assembly in human is organ formation and morphogenisis in embryo. Self-assembly in tissue engineering (TE) could be applied to (i) biomaterial (scaffold) and (ii) cells. Self-assembly could be accomplished in vitro – before implantation of TE construct or/ and in vivo – in the body.
Self-assembly of biomaterials
Let’s look at self-assembly of biomaterials in TE constructs first. There is number of proposed methodologies for biomaterial self-assembly, including embedding peptides, DNA, oligonucleotides, nanoparticles and its induction to assemble by physical, chemical or biochemical interactions. The point of programming biomaterial to self-assemble is to create desired shape (example: cylinder for blood vessel) in different scale (nano-, micro-, macro-), program new physical properties and “help” embedded cells to self-organize in tissue structures in vivo:
The self-assembly method could help solve one of the major challenges in tissue engineering: regrowing human tissue by injecting tiny components into the body that then self-assemble into larger, intricately structured, biocompatible scaffolds at an injury site.
“By using DNA glue to guide gel bricks to self-assemble, we’re creating sophisticated programmable architecture,” says Peng Yin, Ph.D., a Core Faculty member at the Wyss Institute…
“Designing a strategy that leverages the power of self assembly used by living systems to direct construction of tissues from tiny component parts represents an entirely new approach for tissue engineering,” said Don Ingber, M.D., Ph.D., the Wyss Institute’s Founding Director. “Peng and Ali have created an elegant and straightforward method that could permit tissues to be reconstructed from within after a simple injection, rather than requiring major surgery.”
Self-assembly of vascular networks
Proper vascularization is a key for successful TE construct implantation. Programmable biomaterials, scaffold-free bioprinting or induced vascular cellular self-assembly could be applied for creation of vasculature. Gabor Forgacs group is using scaffold-free bioprinting with post-printing fusion to create hollow vascular structures. Seem like fusion could be a common mechanism of self-assembly of vascular structures.
Another example of creation vasculature is substrate-directed self-assembly of vascular cells. It was recently accomplished with vascular mesenchymal cells. Pluripotent stem cells also could be used for creation of 4D vascular networks.
In biological systems multicellular self-organization involves a combination of self-assembly, self-patterning and self-driven morphogenesis. Self-assembly involves the time-evolving control of cell positions relative to each other, such as in the formation of a layered pattern in tissue development. Self-patterning is the spatiotemporal control of cell status, so that cells acquire heterogeneous properties in a region-specific manner from a homegeneous cell population. Self-driven morphogenesis is the spatiotemporal control of intrinsic tissue mechanics…
Cell memory of morphogenesis
Embryonic and mature cells, dissociated from tissue into suspension retain “memory for original tissue morphology”. We can find multiple example of this natural phenomenon in literature. Cell suspension of dissociated intesinal crypts was self-organized in de novo crypts without niche. Embryonic mouse kidneys cells dissociated in suspension can autonomously reaggregate in appropriate culture conditions.
Stem cell self-assembly into “organoids”
In the recent few years, significant amount of work have demonstrated the power of stem cells to recapitulate embryonic organogenesis. As for adult stem cells – single intestinal stem cell can re-create a whole crypt and single mammary stem cell can form mini-mammary gland in vivo. But the most amazing results were recently demonstrated with pluripotents stem cells. Embryonic stem cells can self-organize and form of the optic cup in 3D culture. Cerebral organoids were re-created, based on self-assembly of embryonic stem cells by Japanese and European groups. In order to achieve this self-organization, researchers used supportive matrix (Matrigel) as part of 3D culture.
Decellularized organ-induced cellular self-assembly
Decellularized organs is very promising natural bio-scaffold in TE. It is very tempting to think that once such organ re-cellularized, cells, instructed by matrix, will (i) differentiate and (ii) self-assemble. The recent study gives us a reason to think so.
We show that the seeded multipotential cardiovascular progenitor cells migrate, proliferate and differentiate in situ into cardiomyocytes, smooth muscle cells and endothelial cells to reconstruct the decellularized hearts. After 20 days of perfusion, the engineered heart tissues exhibit spontaneous contractions, generate mechanical force and are responsive to drugs.
We’re entering a new exciting era in tissue engineering – a “4D era”. Self-assembly is natural phenomenon, which we can study and use to re-create organ-like structures for treatment of diseases. Multiple examples of how it could be done, which I attempted to compile in this post, demonstrate the progress in the field of 4D and future prospects.