Holotomography

HT provides 3D dynamic images of live cells and thin tissues without using exogenous labeling agents such as fluorescence proteins or dyes. HT enables quantitative live cell imaging, and also provides quantitative information such as cell volume, surface area, protein concentration. The label-free imaging and quantification of chromosomes were presented.[8] The regulatory pathway of proteasome degradation by autophagy in cells were studies using HT.[9]

HT can be used with other imaging modalities for correlative imaging. For example, a combination of HT and fluorescence imaging enables a synergistic analytic approach.[10][11] HT provides structural information whereas fluorescence signal provides molecular specific imaging, an optical analogous to PET/CT. Various approaches have been reported for correlative imaging approaches using HT.

Intracellular lipid droplets play important roles in energy storage and metabolism, and are also related to various pathologies, including cancer, obesity, and diabetes mellitus. HT enables label-free and quantitative imaging and analysis for free or intracellular lipid droplets. Because lipid droplets have distinctly high RI (n > 1.375) compared to other parts of cytoplasm, the measurements of RI tomograms provide information about the volume, concentration, and dry mass of lipid droplets.[12] Recently, HT was used to evaluate the therapeutic effects of a nanodrug designed to affect the targeted delivery of lobeglitazone by measuring lipid droplets in foam cells.[13]

HT provide various quantitative imaging capability, providing morphological, biochemical, and mechanical properties of individuals cells. 3D RI tomography directly provides morphological properties including volume, surface area, and sphericity (roundness) of a cell. Local RI value can be translated into biochemical information or cytoplasmic protein concentration, because the RI of a solution is linearly proportional to its concentration.[14] In particular, for the case of red blood cells, RI value can be converted into hemoglobin concentration. Measurements of dynamic cell membrane fluctuation, which can also be obtained with a HT instrument, provides information about cellular deformability. Furthermore, these various quantitative parameters can be obtained at the single cell level, allowing correlative analysis between various cellular parameters. HT has been utilized for the study of red blood cells,[15] white blood cells,[16] blood storage,[17] and diabetes.[18]

The quantitative label-free imaging capability of HT have been exploited for the study of various infectious diseases. In particular, parasites-invaded host cells can be effectively imaged and studied using HT. This is because the staining or labeling of parasites requires complicated preparation process and the staining/labeling is not very effective in several parasites. The invasion of plasmodium falciparum, or malaria inducing parasites, to individual red blood cells were measured using HT.[19] The structural and biophysical alteration to host cells and parasites have been systematically analyzed. The invasion of babesia parasites to red blood cells were also studied.[20] Toxoplasma gondii, apicomplexan parasite causing toxoplasmosis, can infect nucleated cells. The alterations of 3D morphology and biophysical properties of T gondii infected cells were studied using HT.[21]

The cell volume and dry mass of individual bacteria or micro algae can be effectively quantified using HT.[22] Because it does not requires the staining process while providing the precise quantification values, HT can be used for testing the efficacy of engineered stains.