A piece of living tissue is a jumble of cells, and even under a microscope it can be hard to distinguish one type of cell from another. In a slice of brain, for instance, the impulse-carrying neurons are all tangled up with starlike support cells called astrocytes. Inside an individual cell the confusion only gets worse. Cells are just crammed with stuff, says Nancy Kedersha, a cell biologist at ImmunoGen, a biotechnology company in Cambridge, Massachusetts. To sort through all that stuff and tell one cell from another, biologists have long resorted to techniques for selectively staining, or color-coding, the structures that crowd their microscope slides. One of the newest and subtlest staining techniques, and the one Kedersha has used to make the pictures shown here, is immunofluorescence.The trick to immunofluorescence is to find a molecule that homes in on a particular protein on or within a cell, and then stain it. Antibodies--immune system molecules that seek out and latch onto infectious invaders in the body--are tailor-made for this job. Laboratory mice, in turn, are tailor-made for making just about any antibody a researcher needs.
To identify neurons in a mixed batch of cells from a rat's brain, for example, Kedersha first injects some of the rat cells into mice. The mice mount an immune response, she explains, and part of the immune response is to make antibodies to various things on the surface of the foreign cells. Once the mice are making antibodies, Kedersha removes their spleens, which is where antibody-producing cells are located. She then sorts through the spleen cells, picks out the ones that make antibodies to the neurons in her original sample, and grows those cells in culture.
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