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Fibrosis, or the pathological scarring of tissue, is the final common pathway for nearly all chronic organ diseases and contributes to approximately 40% of deaths in industrialized nations1. In the liver, fibrosis arises from chronic insults such as viral hepatitis, alcohol use, or increasingly, metabolic dysfunction-associated steatotic liver disease (MASLD), which currently affects 30-40% of adults in the U.S. Chronic insults to the liver and the resulting progressive fibrosis drastically increase the risk of end-stage liver diseases such as cirrhosis, liver failure, and hepatocellular carcinoma2,3,4,5.
Hepatic stellate cells (HSCs) are central to liver homeostasis and fibrogenesis. In healthy livers, HSCs exist in a quiescent, vitamin A-storing state and contribute to the homeostasis of hepatocytes and, overall, the entire liver6,7. This is clearly demonstrated by HSC depletion studies, where the removal of nearly all HSCs in healthy liver decreases liver mass and severely limits liver regeneration7,8. Upon injury, HSCs activate into myofibroblast-like cells that proliferate, migrate to damaged areas, and produce excessive extracellular matrix (ECM), forming fibrotic scars. Activated HSCs in fibrotic livers are also the source of cancer-associated fibroblasts and play tumor-promoting roles in hepatocellular carcinoma9,10,11,12. Quiescent HSCs possess a unique neuron-like morphology with small cell bodies from which long-range projections extend. HSCs shift to a flat, sheet-like myofibroblast shape when activated in fibrosis13. While decades of work have elucidated the signaling pathways controlling ECM production by HSCs, the functional significance of their distinct neuronal morphology, how it is regulated, whether it could facilitate interactions within the HSC niche, and contribute to HSC fibrogenicity and disease remain elusive14,15.
Current methods to visualize HSCs' morphology are limited to two-dimensional (2D) images of either in vitro cultured cells or in vivo tissue sections, and rarely capture the entirety of HSC morphology with projections extending several cell body lengths in all directions. Moreover, the most commonly used HSC marker, desmin, cannot fully capture the unique neuron-like morphology of the HSCs in immunofluorescence staining, as desmin is a cytoskeleton protein that only labels intermediate filaments within HSCs16. These methodological challenges thus far hindered the ability to fully capture HSC morphology and their cellular interactions in a three-dimensional (3D) manner in their natural context at single-cell resolution. As a result, fundamental questions regarding how HSC morphology relates to their functional state and interactions with neighboring cell types have remained unresolved.
This article presents an innovative and comprehensive imaging and analysis pipeline to visualize and model HSC morphology in intact liver tissue. This method integrates several key innovations: (1) fluorescent labeling of HSCs using genetic reporter mouse models, (2) optimized in situ liver perfusion and fixation to preserve intact tissue architecture and cellular relations, (3) a modified iDISCO-based tissue clearing protocol to achieve optical transparency17, (4) high-resolution confocal microscopy for deep-tissue image capture, and (5) a customized computational workflow for 3D image reconstruction and quantitative analysis of individual HSCs. The protocol imposes certain constraints on tissue size and time commitment that need to be taken into consideration. Together, this protocol provides a robust and reproducible framework for capturing the complex 3D structure of HSCs and for mapping their spatial relationships with neighboring vascular, immune, and parenchymal cells. The strategies and pipelines presented here for the liver are readily adaptable to other organs as well17,18,19.