Humans have distinct parts – fingers, toes, arms, legs, organs – thanks to a complex process of cell migration and self-assembly in the early embryo. One of the great mysteries still confronting developmental biologists is that this process is only partly driven by information stored in our genes.
Instead, much of the structure of a developing embryo, including fundamental body segments like the head, thorax, and abdomen, seems to emerge as the result of physical forces between cells and molecular signals that ripple through the embryo like waves. This biological cascade creates self-organized, complex structures “that are represented nowhere in an organism’s genome,” according to Stuart Newman, a cell biologist at New York Medical College in Valhalla, NY.
But recently, biologists have begun to understand – and even manipulate – some of the physical and chemical factors that regulate how cells proliferate, congregate, and separate. Equations usually found in physics textbooks are turning up in tissue-engineering labs, where they’re being used in a radical new process called organ printing.
Tissue engineering is a broad discipline that aims to manufacture viable replacements for damaged or missing body parts such as skin, cartilage, bone, blood vessels, muscles, and livers. One of the field’s pioneers is chemical engineer Robert Langer of MIT, whose lab, in the 1980s, developed a polymer scaffolding to mold into the shape of an organ or a chunk of tissue. Such scaffolds became the most common way to try to build body parts; but over the past six years, researchers have begun to explore organ printing as an alternative to it.
Printing involves depositing layer after layer of cells into a Jello-like support called a hydrogel, a substance that is suffused with nutrients that cells digest and then use to create their own support. The hydrogel physically and chemically mimics the surroundings of embryonic cells during development and allows the cells to migrate as they would naturally. The advantage of organ printing over polymer scaffold-based tissue engineering is that tissues can be built up in days rather than months, and no polymers need to be absorbed by the body -– the hydrogel is made mainly from the same elastins and other molecules found in the extracellular matrix of biological tissue.
“Organ printing can guide the [tissue] assembly,” cell biologist Newman explains. “Instead of leaving it up to the embryo, you set up the initial conditions with an organ printer.” Newman’s team won a $5 million dollar grant from the National Science Foundation last fall to explore the physical mechanisms that help printed organs develop.
One of the fundamental physical principles of tissue development, Newman says, is the Steinberg model, named after Malcolm Steinberg, a well-known Princeton University molecular biologist. Proposed in the 1960s, this model depicts the cells of an embryo as if they were molecules in a liquid. Just as molecules in liquid are bound by molecular forces, cells in tissue are bound by molecules called cadherins. Different tissues have different cells with different binding strengths and they can separate into layers, like oil and water. Consequently, tissue engineers don’t have to worry about meticulously controlling different cell types once they’re deposited, because “they will sort out like salad dressing,” Newman says.
The Steinberg model has been used to drive computer simulations and, most recently, to print tubular structures resembling blood vessels. In 2005, Gabor Forgacs at the University of Missouri in Columbia and colleagues used spherical aggregates of ovary cells from Chinese hamsters, composed of hundreds of the same type of cell. When the cell spheres were printed in a ring formation and stacked on top of one another, they fused together within 24 hours. Forgacs also showed that when two types of cell spheres were used, they separated from each other and formed a tube with two different layers.
In a paper in Physical Review Letters, Forgacs explained that these tubes are precursors to the formation of more complicated structures, such as thick blood vessels, which are composed of two types of cells: endothelial cells line the interior wall and smooth muscle cells are on the exterior.
Understanding what printing process produces the best blood vessels is important for tissue engineering as a whole, says Vladimir Mironov, developmental biologist at the Medical University of South Carolina, who collaborates with Forgacs and Newman. If an organ is printed without a viable vascular system, Mironov says, there is no way to supply nutrients to the tissue, and it will die.
Newman and his colleagues are using animal cells such as chicken cells right now; but their goal is to do the experiments with human cells. “As a scientific tool on animal models, the late-state embryo cells are very desirable experimental models because they will teach us about all sorts of properties of tissues,” he says. One problem, of course, is that human embryonic cells are difficult to obtain and controversial to use, especially embryonic stem cells. But Newman thinks they’ll eventually be able to use less controversial adult stem cells.
Ultimately, for human organ printing, Newman believes that researchers will most likely turn to adult stem cells – cells taken from mature tissue with limited ability to become a specialized cell type -– because they have recently shown promise as an alternative to embryonic stem cells, and they are less controversial. But at the same time that biologists are smoothing out the details of the genetic contribution to development, biophysicists are figuring out the basics of self-assembly in cells. This combined effort could be the ticket to organs on demand.
Home page image courtesy of Vladimir Mironov and Gabor Forgacs.