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Introduction: Cardiac papillary fibroelastomas (CPFs) are rare benign cardiac tumors typically found on the heart valves. The previously published data on the CPF focused on its clinical presentation, optimal management, and prognosis. However, histogenesis of these lesions remains controversial. Accordingly, the aim of this study was to establish the role of endocardial endothelium (EE) in CPF formation., Materials and Methods: Four CPF tumors removed from the right atrioventricular valves were analyzed using hematoxylin & eosin, orcein, and Masson trichrome staining together with immunochemistry for CD-34, CD-68, vimentin, vWF and a-SMA. Moreover, conventional transmission electron microscopy was used for morphological analysis and a-SMA presence confirmation., Results: Ultrastructural morphology, immunohisto- and immunocytochemical analyses indicated that cells covering collagenous core have an endothelial origin. Some endocardial endothelium cells have the potential to undergo a transition to mesenchymal cells. Moreover, the abundant presence of extracellular vesicles may indicate an active intercellular communication. Within the intermediate translucent zone, amorphous substances with monocytes/macrophage-like cells and fibroblastic cells were found. Finally, within collagenous core activated (myo)fibroblasts were observed., Conclusions: Our study demonstrated that the endocardial endothelium of the CPF was "double-sided", i.e., it presented both endothelial and mesenchymal cell characteristics. Another finding was the presence of monocytes, and macrophages which were integrated into CPF core and displayed features of a fibroblast that have been shown to contribute to extracellular matrix production. This could be interpreted as being attributed to the CPF histogenesis.

Over the years Transmission Electron Microscopy (TEM) has evolved into a powerful technique for the structural analysis of cells and tissues at various levels of resolution. However, optimal sample preservation is required to achieve results consistent with reality. During the last few decades, conventional preparation methods have provided most of the knowledge about the ultrastructure of organelles, cells and tissues. Nevertheless, some artefacts can be introduced at all stagesofstandard electron microscopy preparation technique. Instead, rapid freezing techniques preserve biological specimens as close as possible to the native state. Our review focuses on different cryo-preparation approaches, starting from vitrification methods dependent on sample size. Afterwards, we discuss Cryo-Electron Microscopy Of VItreous Sections (CEMOVIS) and the main difficulties associated with this technique. Cryo-Focused Ion Beam (cryo-FIB) is described as a potential alternative for CEMOVIS. Another post-processing route for vitrified samples is freeze substitution and embedding in resin for structural analysis or immunolocalization analysis. Cryo-sectioning according to Tokuyasu is a technique dedicated to high efficiency immunogold labelling. Finally, we introduce hybrid techniques, which combine advantages of primary techniques originally dedicated to different approaches. Hybrid approaches permit to perform the study of difficult-to-fix samples and antigens or help optimize the sample preparation protocol for the integrated Laser and Electron Microscopy (iLEM) technique.