Strategies to visualize cellular membranes with light microscopy are restricted by the diffraction limit of light, which far exceeds the dimensions of lipid bilayers. Here, we describe a method for super-resolution imaging of metabolically labeled phospholipids within cellular membranes. Guided by the principles of expansion microscopy, we develop an all-small molecule approach that enables direct chemical anchoring of bioorthogonally labeled phospholipids into a hydrogel network and is capable of super-resolution imaging of cellular membranes. We apply this method, termed lipid expansion microscopy (LExM), to visualize organelle membranes with precision, including a unique class of membrane-bound structures known as nuclear invaginations. Compatible with standard confocal microscopes, LExM will be widely applicable for super-resolution imaging of phospholipids and cellular membranes in numerous physiological contexts.

Strategies to visualize cellular membranes with light microscopy are restricted by the diffraction limit of light, which far exceeds the dimensions of lipid bilayers. Here, we describe a method for super-resolution imaging of metabolically labeled phospholipids within cellular membranes. Guided by the principles of expansion microscopy, we develop an approach featuring cell-permeable reagents that enables direct chemical anchoring of bioorthogonally labeled phospholipids into a hydrogel network and is capable of tunable, isotropic expansion, thus facilitating super-resolution imaging of cellular membranes. We apply this method, termed lipid expansion microscopy, to visualize organelle membranes with precision, including a unique class of membrane-bound structures known as nuclear invaginations. As it is compatible with standard confocal microscopes, lipid expansion microscopy will be widely applicable for super-resolution imaging of phospholipids and cellular membranes in numerous physiological contexts.

Membrane architectures whose dimensions and features are smaller than the diffraction limit of light orchestrate diverse cellular events such as lipid transport, vesicle formation, and calcium signaling. These structures, which include membrane invaginations, organelle contact sites, and membrane microdomains, are primarily composed of phospholipids, making methods to visualize these biomolecules vital to our understanding of cellular function. Techniques to accurately image phospholipid‐containing structures with fluorescence microscopy are challenged by the diffusion of lipids within the bilayer, even in fixed samples. Expansion microscopy (ExM) utilizes hydrogel formation to fix biomolecules in place and swell samples to produce high‐resolution images of protein‐ and nucleic acid‐containing cellular structures 30‐70 nm in size. Using chemical reporter metabolites and a novel multifunctional fluorophore probe, here we present Lipid Expansion Microscopy (LExM), which enables the high‐resolution imaging of phospholipids with a tunable expansion factor using the principles of ExM. We will present key synthetic and technological advances critical to the development of LExM as well as its application to visualize nanoscale membrane structures, phospholipid‐organelle colocalization, and the spatial component of flux through specific lipid biosynthetic pathways within intact cells.