Single-Molecule Fluorescence In Situ Hybridization for Spatial Detection of mRNAs in Sections of Mammalian Testes

Single-molecule fluorescence in situ hybridization (smFISH) enables the detection and localization of individual mRNAs in tissue sections with single-molecule resolution while preserving spatial context, and thus, is a useful tool for examining gene expression in biological systems. In particular, the growing reliance on single-cell RNA sequencing (scRNA-seq) to explore cellular heterogeneity has reinvigorated this approach as a validation tool to spatially re-map mRNA expression patterns described in isolated cells to their parent tissue. While use of antibody-based methods, such as indirect immunofluorescence (IIF), remain popular as validation strategies, smFISH often affords superior specificity and maintains congruency with scRNA-seq. Here, we present a detailed protocol that combines multiplexed smFISH using the RNAscope approach with IIF to co-visualize mRNAs and proteins within sections of mouse testes. We provide step-by-step guidelines from testis preparation through visualization that enables mapping of combinations of up to four mRNA/protein targets in frozen sections on the RNAscope platform.