The Cell Mechanics

Matthew Holm
December 01, 1995

On the one hand, you have engineering, with die-cast steel turning through measured arcs, while in the distance loom massive spans of reinforced concrete, every angle, every curve, and every pound mathematically accounted for. On the other hand, you have biology, with its fragile, wriggling, willful microstructures, tough to observe, tough to explain, and just plain messy. A casual observer would be hard-put to find a friendly place for the hard edges and soft membranes of these disciplines to meet.

Yet a merging of biology and engineering does not have to mean cyborgs à la Robocop. "We use engineering tools to solve biological problems," explains Jian Cao, a graduate student in bioengineering at Penn State. Cao is one of four graduate students who, under the direction of assistant professor Cheng Dong, are trying to explain in engineering terms the squishy nature of the cell.

"The cell is the basic anatomical unit of life the basic unit for the engineering of living systems," Dong notes. "Yet many cellular functions have not been thoroughly addressed because of the lack of a quantitative understanding of the fundamental properties and interactions involved. By integrating engineering sciences with cell and molecular biology, tissue engineering has the potential to provide such understanding."

Research in Dong's Cellular Biomechanics Laboratory ranges from basic investigations into the immune response, wound healing, tissue development, and tumor metastasis, to such practical problems as how to seed cells on artificial surfaces for use as biological substitutes. One current focus is on the process of cell adhesion.

While most people know that white blood cells fight disease, the process isn't as simple as it sounds. The WBCs that whiz through our blood vessels must first find the infection site, move to the vessel wall, slow down, and attach themselves to the endothelial cells that line the vessel before they can move into the body tissues and destroy germs as Jian Cao says, "They need to adhere first, then dig a hole."

Cao, who did his master's degree work in biomechanics at the University of Akron, and his colleague Xiao Xiao Lei, who received her bioengineering master's degree in China, are studying the mechanical properties of white blood cells and the exact mechanism by which they attach to vessel walls.

And, of course, a problem even more difficult than that of the cell adhering to the vessel wall is trying to observe the cell adhering to the vessel wall. It's not easy to get inside someone's veins. Researchers usually rig up a lab environment similar to a blood vessel, one that allows them to alter flow rates, change the chemical composition of the fluid, and view the adhesion process. They start with a clear tube or syringe, and then lay down either actual endothelial cells (from rats) or the molecules in the endothelial cells to which the white blood cells bond, forming a surface for them to stick to. Through the microscope, the researchers then have a bird's-eye view of the action.

The process of white blood cell adhesion garnered from such top-view flow chambers is as follows: The white blood cell goes merrily through the circulatory system until it detects a chemical signal telling it there is an infection nearby; it then undergoes what is called transendothelial migration it touches down on the vessel wall and crawls through the endothelium until it reaches the infection site, where it sets to work.

Cao and Lei look at the process from a different angle. Literally: They built a side-view flow chamber. From this new vantage point, they discovered that when a white blood cell touched down on the vessel wall, it bounced and rolled like a ball granted, a very sticky one. The cell then began to adhere, the fluid stresses deforming it from its spherical traveling form into a very forward-leaning teardrop shape that had never before been seen from the side.

While others felt that the chemical bonding between the white blood cells and the endothelial cells alone was the key to the adhesion process, Cao and Lei began to examine the effects of cell deformability. Since their observations showed that the white blood cell did not behave like a hard sphere (as had been previously thought) but rather was very soft and deformable, they felt that the amount of surface area in contact with the endothelial cells had to be important. Healthy, normal cells seemed to have the proper deformability rigid white blood cells would have too small a contact area to stop the cell's motion, while excessively deformable white blood cells would have a huge contact area and become glued to the vessel wall, making movement difficult.

After the practical engineering problem of making and using the flow chamber, then, Cao and Lei moved on to the more theoretical task of creating two-dimensional mathematical models of the white blood cells, accounting for all of the forces involved. As if the cell were an airplane in a wind tunnel, they measured things like shear stress against the moving cell, and used a computer to calculate bond forces, surface area, and fluid dynamics. Seeing how the cell deformed, they assumed for modeling purposes that the cell behaved not like a hard ball, but like a thin, elastic shell filled with an incompressible fluid. They modeled the chemical adhesion bonds as springs (to simulate the elastic nature of the bonds), and accounted for the changes in speed and force when the bonds formed and broke as the cell flowed past the surface of the wall.

The goal of all this work is simply a better understanding of white blood cell movement and adhesion. "If we know what affects deformity, we can treat the cell to change its adhesion properties," says Cao. Since the white blood cell's normal stickiness seems to be ideal, such treatment would not be used to "soup up" cells, but rather to restore cells with irregular adhesion properties to normal a tune-up, if you will.

Cell migration is a second focus of study in the Cellular Biomechanics Lab, specifically the frightening migration of cancer cells. Surgeons can often detect and remove tumors, but that will not always stop the spread of cancer through the body individual cancer cells can detach from a tumor, invade the blood vessels, and flow freely throughout the bloodstream (the process called metastasis) until they find a suitable place to alight, then invade new tissue and form a new tumor.

Jun You, who received his master's in applied mechanics in China, and Joe Ciervo, whose master's work was in the Penn State Artificial Heart Lab next door to his current one, are studying the movement of these rogue cancer cells, specifically when they exit blood vessels.

Cancer cells loose in the bloodstream need to get into body tissue in order to settle down and grow into new tumors. The way in lies through the cracks between the endothelial cells that line the blood vessels. The cancer cell, attracted by molecules of collagen emanating from the opening the substrate that lies beneath the endothelial cells is made of the protein collagen extends pseudopodia (literally, "false feet") into the space, attaches itself to the substrate (another example of the closely-related cell adhesion process), and pulls itself through.

In order to simulate the small space between endothelial cells in a controlled lab environment, You and Ciervo use a micropipette with an opening 6.8 microns in diameter. By placing a single melanoma cell in a suspension medium at the opening of the pipette, they can study what conditions affect the formation of pseudopodia, cell migration, and the motility of the cell in general.

In their experiments, You and Ciervo are trying to determine where the protrusion force that allows the pseudopod to thrust out comes from. The environment of the pipette gives them a number of experimental variables to alter and test, including the solution used to simulate blood and the distance between the cancer cell and its collagen target.

Changing the solution surrounding the cell alters the cell's osmotic pressure decrease it, and the pseudopod grows more quickly; increase it, and the pseudopod grows more slowly. The distance from the collagen source controls the concentration gradient of collagen molecules around the cell a greater concentration gradient yields faster cell movement, because more pseudopodia are formed to pull the cell forward.

Pseudopod formation, they feel, works something like this: Inside the cell are long chains (polymers) of actin, which help regulate osmosis; molecules of collagen in the fluid outside the cell trigger the release of calcium, which severs (depolymerizes) the actin polymers; this increases the osmotic pressure within the cell and allows water from outside to flow in and force a pseudopod to push forward. "We're still trying to find out where the calcium comes from," says Ciervo. During protrusion, calcium is present in both the cell and in the surrounding solution, but You and Ciervo have not yet determined whether it comes from outside the cell or from internal stores.

In addition to the calcium problem, You and Ciervo are working out mathematical models that simulate the protrusion process at the molecular level. Once these are complete, they will have a clearer understanding of the specific physics involved in protrusion.

"By just using the engineer's viewpoint, we can apply mathematics to biology and get more quantitative results," You says. "It's a new era for engineering and biology."

Jian Cao, Xiao Xiao Lei, Jun You, and Joe Ciervo are graduate students pursuing Ph.D.'s in bioengineering, 307 Hallowell Building, University Park, PA, 16802; 814-863-1760. Cheng Dong, Ph.D., is assistant professor of bioengineering, College of Engineering, 233 Hallowell Building, University Park, PA, 16802; 814-865-1407. Cao and Lei's project is funded by the National Science Foundation and the Whitaker Foundation. You and Ciervo's is funded by the National Science Foundation and the American Cancer Society.

Last Updated December 01, 1995