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To become better healers, tissue engineering needs a timely
and reliable way to obtain enough raw materials: cells that either already are
or can become the tissue they need to build. In a new study, Brown University
biomedical engineers show that the stiffness, viscosity, and other mechanical
properties of adult stem cells derived from fat, such as liposuction waste, can
predict whether they will turn into bone, cartilage, or fat.
That insight
could lead to a filter capable of extracting the needed cells from a larger and
more diverse tissue sample, said Eric Darling, senior author of the paper
published in Proceedings of the National Academy of Sciences. Imagine a surgeon using such a filter to
first extract fat from a patient with a bone injury and then to inject a high
concentration of bone-making stem cells into the wound site during the same
operation.
In the paper, the
researchers report that the stiffest adipose-derived mesenchymal stem cells
tended to become bone, the ones that were biggest and softest tended to become
fat, and those that were particularly viscous were most likely to end up as
cartilage.
“The results are
exciting because not only do the mechanical properties indicate what lineage
these cells could potentially go along but also the extent of their
differentiation,” said Darling, assistant professor of medical science in the
Department of Molecular Pharmacology, Physiology. and Biotechnology and the
University’s Center for Biomedical Engineering. “It tells us how good they are
going to be if we differentiated them for a given tissue type.”
So when tissue
engineers go looking through extracted fat for cells to create bone, for
instance, they can sort through the cells looking for ones with a certain level
of stiffness or greater. Whether the cells are “undifferentiated” stem cells
that have made no move toward becoming a specific cell type, or ones that are
already bone cells, only the ones with the requisite stiffness would make the
cut. That process would yield a higher population of cells for making new bone
tissue.
“Can we enrich
the cell populations for cells that we want to use, whether they are totally
undifferentiated cell types, partially differentiated, or completely
differentiated?” Darling said. “It doesn’t matter as long as it’s targeted for
the specific tissue application.”
Darling’s study,
led by research assistants Rafael Gonzaelz-Cruz and Vera Fonseca, involved
cloning adipose-derived adult human stem cells into 32 stem cell populations.
They then poked, prodded, and measured the cells with an atomic force
microscope, gaining measurements of how big they were, how sturdy they were
under pressure, and how the force between them and the scope’s cantilevered
probe changed over time. The team found the cells exhibited a wide range of
stiffness, viscosity, and size.
Once they had the
measurements, the researchers chemically induced the cells to differentiate and
analyzed the levels of key metabolites produced by the cells as they matured a
few weeks later. For each population, the metabolites indicated the relative
proportion that differentiated into one tissue or another. Population 28, for
example, apparently responded productively to chemical cues for producing
cartilage, only somewhat well for producing bone and poorly to cues for making
fat.
The key moment
was when the researchers correlated each cell population’s mechanical
measurements with its metabolite data. Did the mechanical properties predict
which populations would be the most successful in turning into bone cells or
cartilage cells or fat cells? Sure enough, they did. The stiffest cell
populations produced more bone. The squishiest cells were the ones that
produced the most fat. The ones with the highest viscosity were the ones
seemingly headed toward becoming cartilage.
Darling and his
team then conducted a sorting simulation to determine whether filtering a
sample by different mechanical characteristics could result in extracting a
higher concentration of useful cells from tissue. They found that while yields
using current methods of looking for molecular biomarkers are less than 1%,
yields based on mechanical properties could increase to 3% for bone-making
cells, 6% for cartilage, and 9% for fat.
Encouraged by the
results, which he acknowledges are basic, Darling’s group is already working
toward a more practical realization of the discovery. After all, surgeons
aren’t going to bring atomic force microscopes into the operating room; those
devices work too slowly to identify enough cells in a reasonable time.
“To actually
apply this, we need some sort of high-throughput mechanical testing device,” he
said. “That is where the next stage is. That’s something that my lab is working
on right now.”
So the idea is
taking shape, like a stem cell striving to become fat, bone, or cartilage.
