The maze is ridiculously simple — something you might find on a child's placemat in a family-friendly restaurant. Yet this maze isn't intended for Crayon-wielding toddlers; it’ has a slightly smaller audience.
For one thing, the maze has more than one solution, with two bifurcated paths of differing lengths. For another, it only measures 1,600-µm2. While that's a tad small for chubby fingers, it's perfect for microscale objects — which, it turns out, is convenient: The maze is a proving ground for epithelial cells.
To the naked eye the maze is perfectly transparent. Under phase-contrast illumination, it appears charcoal gray, with sharply defined passages edged in black and white hinting at a three-dimensional architecture. Fluorescent green cells enter at the bottom and exit at the top. The question is, what route will they take to get there? Some 70% of human mammary epithelial cells chose the short route, and about 5% take the long way ‘round; the rest, well they got lost in the maze. Yet all made their choice without any apparent chemical or physical guidance, a feat you can watch on YouTube (www.youtube.com/watch?v=yzW5GGYk5Co).
These findings demonstrate something interesting about epithelial cells, says Daniel Irimia, the researcher who designed this challenge and an assistant professor in the Surgery Department at Massachusetts General Hospital: In the absence of external cues these cells can navigate via a self-generated gradient, an observation that could help explain how normal epithelial cells move during wound repair, and how cancer cells metastasize.
Irimia is interested in the mechanics and metrics of how cells move: their morphology, speed, directionality, and persistence. The objective, he explains, is to move the science of cell motion from the qualitative to the quantifiable, taking advantage of microf luidic (“lab-on-a-chip”) devices to make precise measurements. After all, only by understanding the normal range of cell behavior — the average speed of an epithelial cell, for instance — can researchers begin to understand how disease impacts that behavior.
“If you think about it, every cell in the body moves or has moved at some point in the lifetime of the individual. There is no cell that is static, and motility is essential for many health and disease processes.”Profiling Neutrophils
One of those processes governs the body's natural defenses. Take, for instance, the systemic immune compromise that often follows severe burns. Patients who suffer burns over significant portions of their body surface area are at heightened risk for infection. Irimia wanted to know understand how that trauma influences a patient's white blood cells.
He and his team designed a microfluidic device from a spongy elastomeric material called PDMS to measure the speed of neutrophils. The device consists of a spacious main channel through which cells move, and an array of tight (3 x 6 micron) chemokine-filled side channels each 1-mm long, like a comb. The goal is to drive the cells into the narrow channels; by forcing the cells to constrict themselves as if they are worming their way through a loose matrix, the chip approximates what happens when neutrophils leave a blood vessel and move towards infected or damaged tissues.
This contrasts the classical method of studying chemotaxis on flat plastic dishes where the cells are free to move in any direction. Under those conditions, Irimia says, cells following a chemotactic gradient tend to engage in a “biased random walk,” like a drunkard staggering home.
“The neutrophil's natural environment is a confined space, where it has to move within physical constraints from every side and still find its way to the target,” he says. Using neutrophils from healthy donors, the team found that the cells confined in narrow channels tend to move at a consistent rate of around 18 microns — about two cell diameters — per minute. When they repeated the experiment using cells from individuals who had been burned over from 10% to 60% of their body, the cells moved more slowly — between 1 and 10 microns per minute — though that speed often recovered over time, correlating with a favorable clinical outcome.
Why the cells move more slowly isn't clear to Irimia — perhaps they are overstimulated and exhausted, he suggests. They may also be a bit punch-drunk: In a separate set of (ongoing) experiments, Irimia and his team designed a microfluidic device that gives neutrophils a choice of routes towards a chemotactic reward, one long and one short. Ninety percent of cells from healthy donors opt for the short route. But cells from burn victims are less sure of themselves sometimes looping through the maze without direction, implying they could have problems reaching damaged tissues in vivo.