Chromosome morphology after long-term storage investigated by scanning near-field optical microscopy

Fluorescence in situ hybridization coupled with far‐field fluorescence microscopy is a commonly used technique to visualize chromosomal aberrations in diseased cells. To obtain the best possible results, chromatin integrity must be preserved to ensure optimal hybridization of fluorescence in situ hybridization probes. However, biological samples are known to degrade and storage conditions can be critical. This study concentrates its investigation on chromatin stability as a function of time following fluorescence in situ hybridization type denaturing protocols. This issue is extremely important because chromatin integrity affects the fluorescence response of the chromosome. To investigate this, metaphase chromosome spreads of human lymphocytes were stored at both ?20 and ?80 °C, and were then imaged using scanning near‐field optical microscopy over a nine month period. Using the scanning near‐field optical microscope's topography mode, chromosome morphology was analysed before and after the application of fluorescence in situ hybridization type protocols, and then as a function of storage time. The findings revealed that human chromosome samples can be stored at ?20 °C for short periods of time (～ several weeks), but storage over 3 months compromises chromatin stability. Topography measurements clearly show the collapse of the stored chromatin, with variations as large as 60 nm across a chromosome. However, storage at ?80 °C considerably preserved the integrity with variations in topography significantly reduced. We report studies of the fluorescent response of stored chromosomes using scanning near‐field optical microscopy and their importance for gaining further understanding of chromosomal aberrations.