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Multiple scattering and absorption limit the depth at which biological tissues can be imaged with light. In thick unlabeled specimens, multiple scattering randomizes the phase of the field and absorption attenuates light that travels long optical paths. These obstacles limit the performance of transmission imaging. To mitigate these challenges, we developed an epi-illumination gradient light interference microscope (epi-GLIM) as a label-free phase imaging modality applicable to bulk or opaque samples. Epi-GLIM enables studying turbid structures that are hundreds of microns thick and otherwise opaque to transmitted light. We demonstrate this approach with a variety of man-made and biological samples that are incompatible with imaging in a transmission geometry: semiconductors wafers, specimens on opaque and birefringent substrates, cells in microplates, and bulk tissues. We demonstrate that the epi-GLIM data can be used to solve the inverse scattering problem and reconstruct the tomography of single cells and model organisms.

Behaviors, such as sleeping, foraging, and learning, are controlled by different regions of the rat brain, yet they occur rhythmically over the course of day and night. They are aligned adaptively with the day‐night cycle by an endogenous circadian clock in the suprachiasmatic nucleus (SCN), but local mechanisms of rhythmic control are not established. The SCN expresses a ~24‐hr oscillation in reduction‐oxidation that modulates its own neuronal excitability. Could circadian redox oscillations control neuronal excitability elsewhere in the brain? We focused on the CA1 region of the rat hippocampus, which is known for integrating information as memories and where clock gene expression undergoes a circadian oscillation that is in anti‐phase to the SCN. Evaluating long‐term imaging of endogenous redox couples and biochemical determination of glutathiolation levels, we observed oscillations with a ~24 hr period that is 180° out‐of‐phase to the SCN. Excitability of CA1 pyramidal neurons, primary hippocampal projection neurons, also exhibits a rhythm in resting membrane potential that is circadian time‐dependent and opposite from that of the SCN. The reducing reagent glutathione rapidly and reversibly depolarized the resting membrane potential of CA1 neurons; the magnitude is time‐of‐day‐dependent and, again, opposite from the SCN. These findings extend circadian redox regulation of neuronal excitability from the SCN to the hippocampus. Insights into this system contribute to understanding hippocampal circadian processes, such as learning and memory, seizure susceptibility, and memory loss with aging.