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The last few decades have slowly witnessed the start of the three-dimensional era, with many innovative approaches developing in the scientific field. For instance, three-dimensional reconstruction approaches, made possible by the development of complex whole‑mount imaging methods paired with high‑resolution fluorescent microscopes, have facilitated a better understanding of several full‑thickness tissue histoarchitectures. These three‑dimensional rendering approaches have been used to define several human body structures, such as vessels, glands, and common tumors4,5,6,7,8,9. Moreover, information on the differences between pathological and normal structures has been collected, for example, in full mouse brains10,11,12. This approach has been identified as a crucial step towards a better understanding of the various biological systems, facilitating a deeper comprehension of cellular circuits, their structural and functional spatial organization, and their potential contribution to human pathologies.
A downside of the different combinations of three‑dimensional tissue reconstruction and imaging is that most approaches involve tissue-clearing steps. Although this method is useful for many tissues, it is not appropriate for all tissue types8,9,19. In fact, clearing procedures involve several washing steps, which could potentially damage targeted epitopes and influence endogenous signals20.
The above‑described approaches have been used to define many important cellular structures within different systems. However, the olfactory system and its structures of interest have a minimal presence of bony components from which they could be easily isolated; therefore, whole‑mount approaches that do not include clearing steps have been trialed in recent years13,14.
These studies recognized the importance of better defining the organization of neuronal structures in the olfactory system. Accordingly, their main focus was to provide a clear topographic map of the olfactory sensory neurons’ distribution based on the expression of different markers14 as well as to reveal novel insights into their peculiar cilia13.
The whole‑mount technique developed and presented here enables the collection and preservation of the mucosal tissue in its entirety, opening the possibility of gaining crucial insights into the anatomical and physiological structure of this environment, with a specific focus, in our case, on the relationship between OECs, fibroblasts, and axon bundles. Pairing this refined isolation and staining technique with high‑resolution confocal imaging enabled the collection of large images that reflect the structure of the entire target region and its resident cells. Specifically, our refined technique offers two distinctive advantages where the entire mucosa is harvested and processed, including the lamina propria architecture13, and the gentler tissue preparation method helps preserve the macro-scale anatomy as well as inter-cellular interactions (such as olfactory ensheathing cells, axons, and fibroblasts) compared to the other flat-mount protocols14. This is also why this protocol does not include validation against thin section staining.
As mentioned above, whole‑mount techniques usually involve long tissue‑processing periods and the use of specific, expensive reagents. This method was developed with a focus on minimal tissue processing to minimize both the timeframe and the risk of damaging important cellular structures. The optimized method, therefore, represents a quick process that can be performed using reagents commonly employed for immunohistochemistry on thin sections, providing a valid alternative to approaches that require longer processing times and are not appropriate for boneless structures such as mucosal tissue.
Image acquisition and post‑processing can be performed using any confocal microscopy platform capable of acquiring high‑resolution z‑stacks from thick tissue samples, with sufficient laser penetration to image through the mucosae of interest (refer to the methods section for more details on our acquisition and image processing settings).
With appropriate modifications, we believe that this protocol can be translated and applied to other common rodent models. With some tissue‑specific modifications, this method can be applied to several different contexts, including the study of mucosal and submucosal tissues from the respiratory, gastrointestinal tract (GIT); the investigation of tissue organization and multi‑system interactions within the nervous system; the analysis of inflammatory processes and mucosa‑associated lymphoid tissues (MALTs); as well as the exploration of different multisystem axes, such as the gut–brain axis.
The main strength of this protocol lies in the methodological refinements that allow for high‑quality three‑dimensional imaging while preserving tissue integrity but also ensuring a relatively fast approach compared to other whole-mount methodologies previously trialed in the field.
The successful implementation of this approach was strongly dependent on a deep anatomical understanding of the tissue of interest. In fact, in our case, knowing the expected localization of the axon bundles was essential to minimize the invasiveness of the collection procedure once the inside of the skull was exposed. This anatomical knowledge was crucial in being able to quickly localize the region and tissue of interest and preserving the overall structure of the mucosa, highlighting how successful tissue processing is intrinsically linked to an informed collection strategy rather than post‑processing steps alone.
As mentioned in the methods section in several notes, multiple technical adjustments were critical to the development and optimization of this method. Maintaining the tissue floating freely within Eppendorf tubes throughout the entire staining process, combined with continuous rotation during reagent and antibody incubation, represented a key step from more static whole‑mount and thin-section protocols. This refinement prevented tissue folding while also ensuring homogeneous reagent penetration across the entire sample, thus improving staining consistency without the need for unnecessary clearing steps. In addition, careful optimization of the incubation duration for each reagent and antibody cocktail proved essential. This last step might need to be further adjusted based on the type of antibody chosen.
Together, these specific factors resulted in enhanced reproducibility while reducing overall processing time and harsher sample handling.
Another important methodological step was the ability to reliably determine tissue orientation prior to mounting and imaging without the need for fluorescent labeling. The lamina propria side of the mucosal tissue could be readily identified under brightfield microscopy by the presence of thick nerve bundles, clearly distinguishable by the surrounding epithelial tissue. Standardizing sample orientation by positioning the lamina propria facing the coverslip side and the epithelial layer resting on the glass slide was found to facilitate image acquisition quality. This simple but effective refinement improved imaging consistency across samples and contributed to the quality of the three‑dimensional reconstructions obtained.
There are some limitations to consider, however. This protocol has not been tested on many common mouse strains or other rodent species. Similarly, the optimizations were conducted under the specific experimental conditions as described above and with the listed antibody panels alone. While the protocol should work with most mouse and rat strains, several different antibodies, and any experimental models that allow for mucosae to be harvested in full thickness, further careful optimization may be required. Another notable limitation is that a single whole-mount section can only be stained with one antibody panel, unlike thin-sectioned tissues, where each section can be stained with individual antibody panel and therefore, multiple different panels of antibodies can be tested on thin-sectioned tissues.
Overall, these methodological innovations demonstrate how relatively straightforward adjustments to tissue handling, staining conditions, and sample orientation can substantially improve whole‑mount imaging outcomes. By avoiding extensive tissue processing and clearing steps, this approach offers a practical and adaptable alternative for the study of delicate, boneless tissues, while still enabling the acquisition of detailed, large‑scale three‑dimensional data.