Mechanical factors are as important as the chemical factors in regulating differentiation of adult stem cells, reports Jianping Fu, Ph.D., an assistant professor in mechanical engineering and biomedical engineering at the University of Michigan. By measuring the traction forces during the culturing process, Dr. Fu and his colleagues were able to predict how the cells would differentiate within 24 hours.
"We show, for the first time, that we can predict stem cell differentiation as early as day one," notes Dr. Fu, who is first author on the paper published August 1 in Nature Methods. "Normally, it takes weeks or maybe longer to know how the stem cell will differentiate. Our method could provide early indications of how the stem cells are differentiating and what cell types they are becoming under a new drug treatment."
The study was conducted in the group of Christopher Chen, M.D., Ph.D., in the department of bioengineering at the University of Pennsylvania. The paper is titled "Mechanical regulation of cell function using geometrically modulated elastomeric substrates."
Dr. Fu’s team examined the slight forces that stem cells exert on the materials they are attached to. These traction forces were suspected to be involved in differentiation, but they have not been as widely studied as the chemical triggers. The researchers were able to show that the stiffness of the material on which stem cells are cultivated in a lab does, in fact, help to determine what type of cells they turn into.
The group started by building a stem cell scaffold, whose stiffness could be tweaked without changing chemical composition. This cannot be done with conventional stem cell growth matrices, Dr. Fu points out. The new scaffold resembled an ultrafine carpet of microposts, hair-like projections made of the elastic polymer polydimethylsiloxane.
By adjusting the height of the microposts, the researchers were able to adjust the rigidity of the matrix. Using human mesenchymal stem cells, they found that the cells differentiated into bone when grown on stiffer scaffolds and into fat when grown on more flexible scaffolds.
Once the investigators observed the cells differentiating according to the mechanical stiffness of the substrate, they decided to measure the cellular traction forces throughout the culturing process to see if they could predict how the cells would differentiate. Using a technique called fluorescent microscopy, the research team measured the bending of the microposts to quantify the traction forces.
"Our study shows that if the stem cells determine to differentiate into one cell type then their traction forces can be much greater than the ones that do not differentiate or that differentiate into another cell type," Dr. Fu explains. "We prove that we can use the evolution of the traction force as early indicators for stem cell differentiation."
The new matrix, manufactured through an inexpensive molding process, is cheap to make, and the University of Michigan team is reportedly giving it away to any interested scientists or engineers. "We think this toolset provides a newly accessible, practical methodology for the whole community," says Dr. Fu.