Project Summary/Abstract Imaging is an important diagnostic and research tool in hematology. This R21 proposal, written in response to PAR-20-140 (Catalytic Tool and Technology Development in Kidney, Urologic, and Hematologic Diseases), is designed to optimize, validate, and deploy a groundbreaking light microscopy platform for visualization of proteins in their ultrastructural context in hematology. Visualizing specific proteins of interest in their ultrastructural (sub-organelle, ~sub-50 nm) context currently requires correlative light and electron microscopy (CLEM) which is extremely complicated, time-consuming and expensive and thereby inaccessible to most researchers in hematology. Pan Expansion Microscopy (PanExM) is a novel imaging approach that allows for imaging of proteins (and other biomolecules and organelles) in their ultrastructural context on a whole-cell basis with nanoscale (~10-30 nm) resolution using conventional (diffraction-limited) light microscopes. With PanExM, fixed cells are expanded up to 20-fold in the linear dimension (~8000 fold in volume) while maintaining subcellular organization, cellular protein content and antigenicity. Staining the preserved ultrastructure as well as the protein of interest in these samples in different colors and imaging them in three dimensions using standard light microscopy allows for visualization and analysis of what would otherwise be below the diffraction limit of light. Aim 1 is focused on developing and optimizing the methodology to expand individual blood and bone marrow cells adhered or immobilized on slides. Aim 2 is focused on optimizing expansion of bone marrow sections to assess hematopoietic cells in their microenvironment. Our highly synergistic research team is dedicated to training and dissemination of the latest technologies in hematology research. Application of this powerful new technology to blood and bone marrow cells will be transformative by enabling hematology investigators to quantify the location, distribution and interactions of molecules of interest in their sub-cellular context. By extending these high-resolution imaging capabilities from a handful to hundreds of research groups and increasing the throughput from weeks per data set to hours, PanExM will democratize the hematology research community’s access to 3D structural information and will drive discoveries across biomedical research ranging from fundamental cell biological findings to histological applications in clinical pathology.