The process that causes living cells to club together to create tissues is driven by both biochemistry and thermodynamics, according to a new study by an international team of scientists. The group’s experiments and computer simulations could help scientists improve technologies for creating artificial tissues and organs.
Multicellular organisms from simple worms to complex mammals comprise tissues and organs that form via the organization of many single cells. This alignment of cells is driven by several processes, some that are biochemical and others that are related to cell-to-cell contact and other interactions with cell exteriors. While cellular alignment processes often have miniscule effects on individual cells, collectively they play a crucial role in the formation and health of tissues.
Alignment often occurs in response to the anisotropy of the cells’ environment, and this results in the migration of cells along a specific direction. This is called “contact guidance” and plays important roles in both tissue growth and tissue homeostasis – the latter being the process by which tissue is maintained in a steady state. While scientists know that contact guidance is important, the underlying mechanism has been poorly understood until very recently.
Biochemistry versus entropy
Now, researchers in the UK, Netherlands, Iran, and Italy have shown that contact guidance can be driven by both biochemical and entropy related processes, depending on the degree to which the cells are confined in an anisotropic environment. Led by Vikram Deshpande at the University of Cambridge, the team placed human muscle cells (myofibroblasts) on substrates containing micropatterned channels made of fibronectin. This is a large glycoprotein that makes up the extracellular matrix of tissues. As well as mediating cell interactions, it also plays roles in cell adhesion, growth and migration.
The cells were placed on the substrates at low densities so that cell-to-cell contact was avoided. The cells measure about 160 micron across and the team observed their behaviour in channels of three different widths – 50, 160 and 390 micron.
The team found that cells in the narrower channels were aligned more than those in wider strips. In the narrower strips, the team concluded that contact guidance occurred because the cells must change their shapes and energy to adjust to the narrower environment — processes that are driven by the biochemical processes within the cells.
What is happening is a little bit counterintuitive
Vikram Deshpande
What surprised the scientists, however, is that contact guidance also occurred in channels much wider than the size of the muscle cells. In this case, the researchers say that the process is driven by an increase in entropy – the thermodynamic tendency of the system to move towards disorder.
“What is happening is a little bit counterintuitive,” explains Deshpande, “You can think that in an aligned system is not maximally disordered, but actually in this case, the maximally aligned system is the most disordered one”.
He says that the phenomenon can be understood by imagining a few matches in a matchbox. If you shake the matchbox, instead of taking a random orientation, the matches would align themselves along the edges of the box. Analogously, cells aligned along the anisotropy of their environment represent a system with a higher entropy.
“There are certain factors that you can experimentally measure, such as the traction, or investigate the shapes to look at the size of their cytoskeletal arrangements. But there are certain features in understanding cellular behavior that are not directly measurable,” added Deshpande. “This is why we also simulated the Gibbs free energy of the cells, to go beyond the experiments.”
Critical width
The team combined the analysis of the cells’ shapes with a statistical analysis of their fluctuations not related to temperature. The resulting model also predicted, that upon increasing the channel width above a certain critical value, the cell orientation would not be driven by its internal biochemistry, but rather by entropy.
The results could have important implications for healthcare, medicine and tissue engineering – which could be achieved by manipulating the shape and organization of cells by changing the geometry of their environment. A better understanding of contact guidance could also help doctors predict the spread of diseases such as metastasizing cancer.
Human immunity is perhaps a touchy-feely process
While the experiment was done on a flat 2D surface, the team is already working on expanding their research to encompass more life-like conditions. “In lots of cases inside the body, surfaces are not flat and the cells are not growing on a flat surface either,” says Deshpande. “We are really interested in understanding how effectively curvature is a driving cue for guiding cells and why do different kinds of cells respond differently to various surfaces and curvatures.”
According to Patrick McGarry from the National University of Ireland Galway, this study “provides a ground-breaking insight into the thermodynamics of biological cells”. McGarry, who was not involved in the research adds, “The seminal finding that entropy is a key driver of cell alignment is fundamental to the assembly and function of living tissue and has highly important implications for the field of regenerative medicine”. He adds, “The work provides a new paradigm for the fusion of thermodynamics, biology, and computational mechanics, leading to a new understanding of the active response of living matter to the surrounding physical environment”.
The results are reported in the Biophysical Journal.
