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And she showed that the paraformaldehyde used to prepare samples for standard microscopy introduces its own artifacts: Applying it causes the pearls to disappear.

Four days later the cultivated embryos were preserved in paraformaldehyde and shipped back to Earth, where a team led by molecular biologist Teruhiko Wakayama of the University of Yamanashi studied the results.

Over time, paraformaldehyde forms bonds between the components that make up neurons, turning the cells into a gel, says neuroscientist María Llorens-Martín of the Severo Ochoa Molecular Biology Center in Madrid.

Last year’s Nature study, for example, looked for new neurons in 59 samples of human brain tissue, some of which came from brain banks where samples are often immersed in the fixative paraformaldehyde for months or even years.

The number of cells that test positive for DCX in brain tissue declines sharply after just 48 hours in a paraformaldehyde bath, Llorens-Martín and her colleagues report today in Nature Medicine.