Histogel-based techniques for embedding organoids in paraffin blocks enable high throughput downstream histopathological analyses

Organoids are <i>in vitro</i> tissue models derived from human or animal primary tissues or stem cells that allow for studying three-dimensional (3D) tissue biology, toxicity testing, biomarker evaluation, and assessment of compound efficacy, supplementing or potentially minimizing use of animal models. Organoids are typically cultured in a 3D format within an extracellular matrix and, at the end of an experiment, can be further processed for various cellular or molecular readouts. Analysis often relies on whole mount immunolabeling for markers of interest, which consumes the entire sample/well, thereby limiting sample availability for downstream assays. In addition, 3D cultures become more friable after fixation and are susceptible to sample loss during washing steps. In contrast, by fixing and processing organoids to a paraffin block, dozens or hundreds of unstained slides can be generated, enabling robust characterization via multiple assays, including histologic evaluation and (immuno)histochemical stains, thus maximizing the yield of these time- and labor-intensive cultures. Here we describe three methods to process 3D Matrigel cultures into paraffin blocks using Histogel as an embedding agent. The three techniques all yield high-quality sections but vary in complexity of implementation at different steps, and their application for different use cases is discussed.