Label-free imaging of chromatin in the live cell nucleus by scattering-based interference microscopy

Yi-Teng Hsiao^1*, Chia-Ni Tsai¹, Chia-Lung Hsieh¹

¹1Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, Taipei, Taiwan

* Presenter:Yi-Teng Hsiao, email:tonyhsian@gmail.com

Chromatin is a molecular complex of DNA and proteins whose organization and dynamics play important roles in the modulation of cell nuclear processes. Current chromatin imaging mainly relies on fluorescence imaging because of its high molecular specificity through labeling. However, the labeling process, e.g., fixation, and the properties of the fluorophores, e.g., photobleaching, have limited its applicability to continuous observation. These problems can be overcome by label-free phase-sensitive optical microscopy through measuring the scattered field of the sample. Unfortunately, the label-free approaches generally lack the molecular specificity of imaging. In this work, we demonstrate a method of using the scattering signal of molecular fluctuation to reconstruct a label-free nucleus image with molecular specificity. A highly sensitive contrast-enhanced coherent brightfield (COBRI) microscopy is used to measure the dynamic scattering signal of chromatin fluctuation at a high speed. Through proper temporal data analysis, the chromatin organization is successfully reconstructed. The label-free chromatin map is highly correlated with the histone fluorescence image. Importantly, the absolute intensity of the label-free chromatin map also reflects the level of molecular compaction state of chromatin. Using our methods, we observe previously undescribed transient chromatin condensation events at the nanoscale. Label-free scattering-based interference imaging offers the opportunity to investigate the nuclear dynamics in the native form and with the unprecedented spatiotemporal resolutions.

Keywords: label-free, interference, coherent brightfield microscopy, molecular fluctuation, chromatin