= DNA synthesis labeling with EdU
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= EdU labeling
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<EdU labeling> uses a modified <nucleoside>, 5-ethynyl-2'-deoxyuridine, incorporated into newly synthesized <DNA>, then chemically couples it to a fluorescent reporter.
A 2008 experiment injected adult mice with $100\,\mathrm{\mu g}$ of this label and inspected intestinal sections 96 hours later. Fixed sections on slides received $10\,\mathrm{\mu M}$ Alexa568-azide for 10 minutes. Imaging with a <fluorescence microscope> combined the reporter image with a Hoechst <DNA> counterstain and a transmitted-light view. Labeled <cell nuclei> marked descendants of <biological cells> synthesizing <DNA> during exposure, rather than identifying current <S phase> at collection. <A chemical method for fast and sensitive detection of DNA synthesis in vivo by Adrian Salic and Timothy J. Mitchison>[A chemical method for fast and sensitive detection of DNA synthesis in vivo, Figure 4 legend: mouse pulse, section preparation and imaging]{ref}
The same study's cultured-cell controls compared overnight $10\,\mathrm{\mu M}$ label, no label, and label with $10\,\mathrm{mM}$ <hydroxyurea>. No-label samples had little reporter background; <hydroxyurea> strongly suppressed incorporation. The camera detected residual signal with a 3-second exposure versus 40 milliseconds under otherwise identical settings, demonstrating why acquisition settings matter. These controls support synthesis-dependent labeling, without proving that every labeled <biological cell> completes division. <A chemical method for fast and sensitive detection of DNA synthesis in vivo by Adrian Salic and Timothy J. Mitchison>[Figure 2 legend, A–M: NIH 3T3 controls, fluorescence and differential interference contrast imaging, exposure-time comparison]{ref}
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