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The recent development of 3D tissue spheroids aims to address current limitations with traditional 2D cell cultures in various studies, including cancer drug screening and environmental toxin testing. In these studies, measurements of cellular viability are commonly utilized to assess the effects of drug or toxins. Existing methods include live/dead assays, colorimetric assays, fluorescence calcium imaging, and immunohistochemistry. However, those methods involve the addition of histological stains, fluorescent proteins, or other labels to the sample; some methods also require sample fixation. Fixation-based methods preclude the possibility of longitudinal study of viability, and confocal fluorescence imaging-based methods suffer from insufficient delivery of labels near the center of 3D spheroids. Here, we demonstrate the use of label-free optical coherence tomography (OCT) for quantitative cellular viability imaging of 3D tissue spheroids. OCT intensity and decorrelation signals acquired from neurospheroids exhibited changes correlated with cellular viability as manipulated with ethanol. Interestingly, when we repeated the imaging while cells gradually became less viable, the intensity and decorrelation signals exhibited different time courses, suggesting that they may represent different cellular processes in cell death. More quantitative measurements of viability using dynamic light scattering optical coherence microscopy (DLS-OCM) will be also presented. DLS-OCM enables us to obtain 3D maps of the diffusion coefficient, and we found that the diffusion coefficient of intra-cellular motility correlated with cellular viability manipulated by changes in temperature and pH. Finally, applications of these novel methods to human-cell 3D spheroids will be discussed.
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