Refractive-index tomography of opaque tissue from its own backscattered light

The refractive index (RI) is an intrinsic, label-free marker of a living cell's dry mass and subcellular morphology, and hence of its physiological state. Its three-dimensional (3D) reconstruction has become a powerful way to study cells and tissues in their native state, spanning cell growth, drug response and disease diagnosis. Yet this capability rests on a fundamental constraint: the RI can be recovered only from light transmitted through the specimen, which demands optical access to both sides. The cells that matter most -- those within thick tissues, intact organs and living animals -- are therefore out of reach. A tissue, however, can illuminate its own cells from behind: light backscattered by intrinsic tissue structures beneath a cell carries the same transmission information a microscope would collect from the far side. Here we develop a divide-and-conquer inverse-scattering framework that recovers this transmission from the backscattering and reconstructs a cell's 3D RI. We demonstrate label-free, quantitative imaging of cells within an engineered tissue, and a living mouse through its intact skull, where we further quantify the dry mass of individual osteocytes in vivo. By removing the need for two-sided access, this reflection-only approach extends RI tomography into living tissue, enabling non-destructive, longitudinal imaging of cells in their native environment.