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By David Brown
Washington Post Staff Writer
Monday, April 7, 2008
Learning to tell things apart and sort them into different piles is a key cognitive milestone of early childhood. It's probably not a surprise that the skill comes in handy all through life and in all manner of activities.
In biology, sorting and piling is the essential first task for studying the differences between things that are, in most ways, similar to each other. People have done that with whole organisms -- beetles, mushrooms, mice -- for thousands of years.
Only in the past 50 years, however, have people been able to perform the task on the building blocks of life, namely cells.
That capability, achieved through devices called cell sorters and flow cytometers, has turned out to be one of science's great technological advances. With unprecedented insights on the molecular basis of disease and the rise of "personalized medicine," cell sorting is heading into its golden age.
Some forms of high-speed cellular identification and sorting are used to do CD4-cell counts in patients infected with the AIDS virus; in diagnosing leukemia and lymphoma; and in helping physicians decide what drug may best treat certain types of cancer. The technology is also used for innumerable tasks that have not yet gotten to the clinic, the best-known being the isolation of stem cells from blood and amniotic fluid.
In some cases, identifying and counting the cells is the work to be done. At other times, the cells must also be physically separated and captured for future use.
Finding and sorting a few cells from a great mass of them is sometimes likened to finding a needle in a haystack. But the job is more like watching a parade of 10,000 undertakers, spotting the two who are wearing yellow ties and the three wearing pink ones, and then getting them out of the line without making everyone else lose step.
How a machine capable of doing that came about is a story that entails all the forces of science: brains, work, timely observation and intellectual cross-fertilization.
The ancestral device was a machine for counting blood cells developed by an engineer named Wallace H. Coulter in 1949.
The principle: If a stream of cells traveling single file in salt water was sent through a tiny orifice and an electric current was directed across the opening, the cells could be distinguished by size based on how much they impeded the current. The Coulter Counter, patented in 1953, is still used today in hospitals to count red cells, lymphocytes, monocytes and other cells in blood.
Figuring out how to sort cells once they were detected was the accomplishment of a physicist named Mack J. Fulwyler.
He was working at Los Alamos National Laboratory in New Mexico on ways to measure radioactive fallout when the atmospheric nuclear test-ban treaty of 1963 suddenly lightened his workload. He began to help biologists at the lab and took up the problem of isolating different types of cells.
He considered using valves or gates to divert the portion of a stream carrying a just-detected cell. But it was quickly clear that that could not be done with enough speed. About this time, Fulwyler read a paper by a Stanford engineer, Richard G. Sweet.
Sweet had discovered a way to create a highly ordered stream of droplets by directing liquid through a tube and out of an orifice oscillating about 30,000 times per second. This led to his invention, the inkjet printer.
Fulwyler realized Sweet had "a way to move small amounts of liquid very rapidly," he recalled in an oral history collected by the Smithsonian Institution in 1991. Moreover, Fulwyler said he thought the "small droplets could then be charged and individually deflected," pulled out of a stream that itself had been partitioned into hundreds of pieces.
Combined with a Coulter-like way of detecting the presence of a cell, Fulwyler said he thought he could come up with "a means of moving a cell around." In August 1964, he visited Sweet, borrowed some inkjet devices, returned to Los Alamos and went to work.
"Mack Fulwyler was the inventor of the cell sorter. There is no question about it," said J. Paul Robinson, a professor of immunopharmacology and biomedical engineering at Purdue University.
Fulwyler's original 1965 cell sorter is long gone. But a copy of it, built in 1967 and sold to a researcher at Brown University, was rescued in 2005 by Robinson just before it was going to be dismantled. It is now at Purdue, but Robinson said he has "absolutely no doubt it will end up in the Smithsonian."
The last big contribution was by Leonard A. Herzenberg, a Stanford biochemist and immunologist. Herzenberg visited Los Alamos in the late 1960s and asked Fulwyler to consider adding a fluorescence detector to the cell sorter. "I said fluorescence would make it very useful to biology," he recalled. Many biological substances (including DNA) are fluorescent in certain wavelengths of light. This would be a way to distinguish cells by something other than size.
In fact, Los Alamos researchers were experimenting with fluorescence detectors on the cell sorter. But it was Herzenberg and his collaborators who over the next dozen years perfected the use of fluorescent dyes and monoclonal antibodies to label cells so that even those virtually identical by size and shape could be differentiated and separated.
Fulwyler died in 2001. In 2006, Herzenberg in 2006 won the $450,000 Kyoto Prize, given to those who have "contributed significantly to the scientific, cultural, and spiritual betterment of mankind."
But the story isn't over. Last month, Massachusetts Institute of Technology researchers published a paper in Nano Letters describing a new approach to cell sorting.
Robert S. Langer and his colleagues use nanotechnology to build a surface covered with various molecular receptors, across which they roll cells. The receptors bind and release the cells at different rates, separating them into collectible groups.
Whether this leads to a better mousetrap -- er, cell trap -- remains to be seen.
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