Quantitative imaging of intracellular density with ratiometric stimulated Raman scattering microscopy

Abstract

Cell size and density impact a wide range of physiological functions, including tissue homeostasis, growth regulation, and osmoregulation. Both are tightly regulated in mammalian cells. In comparison, density variation of a given cell type is much smaller than cell size, indicating that maintenance of cell type-specific density is important for cell function. Despite this importance, little is known about how cell density affects cell function and how it is controlled. Current tools for intracellular cell density measurements are limited either to suspended cells or cells growing on 2D substrates, neither of which recapitulate the physiology of single cells in intact tissue. While optical measurements have the potential to measure cell density in situ and noninvasively, light scattering in multicellular systems prevents direct quantification. Here, we introduce an intracellular density imaging technique based on ratiometric stimulated Raman scattering microscopy (rSRS). It quantifies intracellular drymass density through vibrational imaging of macromolecules. Moreover, water is used as an internal standard to correct for aberration and light scattering. We demonstrate real-time measurement of intracellular density quantification and show that density is tightly regulated across different cell types and can be used to differentiate cell types as well as cell states. We further demonstrate dynamic imaging of density change in response to osmotic challenge as well as intracellular density imaging of a 3D tumor spheroid. Our technique has the potential for imaging intracellular density in intact tissue and understanding density regulation and its role in tissue homeostasis.

Single-cell monitoring of dry mass and dry mass density reveals exocytosis of cellular dry contents in mitosis

“…Dry mass measurements are critical for understanding cell size and growth regulation, but they are often limited in resolution and, more importantly, they do not inform us about the composition of the dry mass. One approach that overcomes this is stimulated Raman scattering microscopy, which can measure the total amount of proteins and lipids in a live cell ( Figueroa et al, 2021 ; Oh et al, 2020 ). While Raman scattering-based methods provide spatial resolution and details about the molecular changes taking place, they can also cause phototoxicity, which significantly limits the temporal resolution when tracking the same cell(s) over time ( Zhang et al, 2021 ).…”