Electron Microscopic Study Using Formalin-fixed, Paraffin-embedded Material, with Special Reference to Observation of Microbial Organisms and Endocrine Granules

Ultrastructural cardiac pathology: the wide (yet so very small) world of cardiac electron microscopy

Electron microscopy (EM) was a popular diagnostic tool in the 1970s and early 80s. With the adoption of newer, less expensive techniques, such as immunohistochemistry, the role of EM in diagnostic surgical pathology has dwindled substantially. Nowadays, even in academic centers, EM interpretation is relegated to renal pathologists and the handful of (aging) pathologists with experience using the technique. As such, EM interpretation is truly arcane-understood by few and mysterious to many. Nevertheless, there remain situations in which EM is the best or only ancillary test to ascertain a specific diagnosis. Thus, there remains a critical need for the younger generation of surgical pathologists to learn EM interpretation. Recognizing this need, cardiac EM was made the theme of the Cardiovascular Evening Specialty Conference at the 2023 United States and Canadian Academy of Pathology (USCAP) annual meeting in New Orleans, Louisiana. Each of the speakers contributed their part to this article, the purpose of which is to review EM as it pertains to myocardial tissue and provide illustrative examples of the spectrum of ultrastructural cardiac pathology seen in storage/metabolic diseases, cardiomyopathies, infiltrative disorders, and cardiotoxicities.

Unique prenatal manifestations of biallelic NDUFAF5 variants: expansion of phenotype

OBJECTIVE

Mitochondrial complex-I deficiency, nuclear type 16, is a rare autosomal recessive disorder caused by biallelic pathogenic variants in NDUFAF5 (C20orf7) (OMIM 618238). The aim of this study was to describe a severe early prenatal manifestation of this disorder, which was previously considered to occur only postnatally.

METHODS

This was a multicenter retrospective case series including five fetuses from three non-related families, which shared common sonographic abnormalities, including brain cysts, corpus callosal malformations, non-immune hydrops fetalis and growth restriction. Genetic evaluation included chromosomal microarray analysis and exome sequencing. Two fetuses from the same family were also available for pathology examination, including electron microscopy.

RESULTS

Chromosomal microarray analysis revealed no chromosomal abnormality in any of the tested cases. Trio exome sequencing demonstrated that three affected fetuses from three unrelated families were compound heterozygous or homozygous for likely pathogenic variants in NDUFAF5. No other causative variants were detected. The association between NDUFAF5 variants and fetal malformations was further confirmed by segregation analysis. Histological evaluation of fetal tissues and electron microscopy of the skeletal muscle, liver, proximal tubules and heart demonstrated changes that resembled postmortem findings in patients with mitochondrial depletion disorders as well as previously undescribed findings.

CONCLUSIONS

Mitochondrial complex-I deficiency and specifically biallelic mutations in NDUFAF5 have a role in abnormal fetal development, presenting with severe congenital malformations. Mitochondrial complex-I disorders should be considered in the differential diagnosis of corpus callosal malformations and brain cysts, especially when associated with extracranial abnormalities, such as fetal growth restriction and non-immune hydrops fetalis. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.

Fluid preservation in brain banking: a review

Fluid preservation is nearly universally used in brain banking to store fixed tissue specimens for future research applications. However, the effects of long-term immersion on neural circuitry and biomolecules are not well characterized. As a result, there is a need to synthesize studies investigating fluid preservation of brain tissue. We searched PubMed and other databases to identify studies measuring the effects of fluid preservation in nervous system tissue. We categorized studies based on the fluid preservative used: formaldehyde solutions, buffer solutions, alcohol solutions, storage after tissue clearing, and cryoprotectant solutions. We identified 91 studies containing 197 independent observations of the effects of long-term storage on cellular morphology. Most studies did not report any significant alterations due to long-term storage. When present, the most frequent alteration was decreased antigenicity, commonly attributed to progressive crosslinking by aldehydes that renders biomolecules increasingly inaccessible over time. To build a mechanistic understanding, we discuss biochemical aspects of long-term fluid preservation. A subset of lipids appears to be chemical altered or extracted over time due to incomplete retention in the crosslinked gel. Alternative storage fluids mitigate the problem of antigen masking but have not been extensively characterized and may have other downsides. We also compare fluid preservation to cryopreservation, paraffin embedding, and resin embedding. Overall, existing evidence suggests that fluid preservation provides maintenance of neural architecture for decades, including precise structural details. However, to avoid the well-established problem of overfixation caused by storage in high concentration formaldehyde solutions, fluid preservation procedures can use an initial fixation step followed by an alternative long-term storage fluid. Further research is warranted on optimizing protocols and characterizing the generalizability of the storage artifacts that have been identified.

Identifying Active Progeny Virus Particles in Formalin-Fixed, Paraffin-Embedded Sections Using Correlative Light and Scanning Electron Microscopy

Immunohistochemical analysis of formalin-fixed paraffin-embedded (FFPE) tissue blocks is routinely used to identify virus-infected cells. However, detecting virus particles in FFPE sections using light microscopy is difficult because of the light diffraction resolution limitations of an optical microscope. In this study, light microscopy and field emission scanning electron microscopy were performed to observe 3-dimensional virus particles in FFPE sections in a nondestructive manner using NanoSuit or osmium conductive treatment methods. The virus particles in FFPE sections were immunostained with specific antibodies against the surface antigens of the viral particles and stained with 3,3'-diaminobenzidine. A metal solution (0.2% gold chloride or 2% osmium tetroxide) was applied to enhance the 3,3'-diaminobenzidine-stained area. This procedure is nondestructive for FFPE sections and is a simpler method than transmission electron microscopy. To validate the applicability of this technique, we performed 3-dimensional imaging of the virus particles of different sizes, such as human papillomavirus, cytomegalovirus, and varicella-zoster virus. Furthermore, ultrathin sections from the FFPE sections that were observed to harbor viral particles using field emission scanning electron microscopy were prepared and assessed using transmission electron microscopy. In the correlative areas, transmission electron microscopy confirmed the presence of large numbers of virus particles. These results indicated that the combination of marking viral particles with 3,3'-diaminobenzidine/metal staining and conductive treatment can identify active progeny virus particles in FFPE sections using scanning electron microscopy. This easy correlative imaging of field emission scanning electron microscopy of the identical area of FFPE in light microscopy may help elucidate new pathological mechanisms of virus-related diseases.

Genomic integration and expression of Felis catus papillomavirus type 2 oncogenes in feline Merkel cell carcinoma

The involvement of Felis catus papillomavirus type 2 (FcaPV2) in feline Merkel cell carcinoma (MCC) has been previously hypothesized. In this study, the expression and localization of FcaPV2 oncogene mRNA, the integration of FcaPV2 genes, and p53 mutations in feline MCC were examined by RNAscope in situ hybridization (ISH), whole genome sequencing (WGS), and Sanger DNA sequencing, respectively. Furthermore, the morphological and molecular characteristics of FcaPV2-positive (FMX-MCC01) and FcaPV2-negative (AS-MCC01) MCC cell lines were compared in vitro and in vivo using immunofluorescence, ISH, xenotransplantation into mice, and immunohistochemistry. ISH for FcaPV2 E6/E7 detected viral RNA in 18/21 FcaPV2-positive MCC and not in 1/1 FcaPV2-negative MCC. WGS of 2 FcaPV2-positive cases revealed the integration of FcaPV2 genes in both cases. In cultured cells and xenograft tissues of FMX-MCC01, most cells were positive for E6/E7 by ISH and p16CDKN2A, a few cells were positive for the retinoblastoma protein (pRb), and all cells were negative for p53. In cultured cells and xenograft tissues of AS-MCC01, all cells were negative for p16CDKN2A, most cells were positive for pRb, and some cells were positive for p53. Missense mutations in p53 were identified in 8/10 FcaPV2-positive and 1/1 FcaPV2-negative MCC. These results suggest that the expression of integrated FcaPV2 oncogenes might be associated with reduced expression of the tumor suppressor proteins pRb and p53 and might contribute to the development of feline MCC. On the other hand, p53 mutations may be involved in both FcaPV2-positive and FcaPV2-negative MCC tumorigenesis.

Periprosthetic joint infection due to Mycoplasma hominis in a multiple sclerosis patient treated with fingolimod

Mycoplasma hominis is a commensal pathogen normally found in urogenital tract of humans and has been associated with a wide variety of extra-genitourinary infections, such as mediastinitis, bacteremia, and septic arthritis, particularly in immunocompromised patients. Here, we present a case of a 48-year-old male, who had been treated with fingolimod for relapsing multiple sclerosis and presented with fever and right-sided hip pain following total hip arthroplasty. CT scan revealed localized fluid collection in the right quadriceps femoris muscle adjacent to the joint cavity of right hip. The percutaneously aspirated fluid grew M. hominis, which was also isolated from blood culture. With diagnosis of periprosthetic joint infection, the patient underwent surgical debridement with retained prosthesis and was treated with antimicrobial agents. Infected granulation tissues excised from the hip was observed under an electron microscope, which revealed electron-dense rounded structures contained in neutrophils, consistent with Mycoplasma particles. Fingolimod, an immunomodulatory drug that acts on the sphingosine-1-phosphate receptor and prevents the egress of lymphocytes from lymph nodes, might increase host susceptibility to a systemic M. hominis infection.

Clinicopathological Correlation of Microbial Keratitis and Ahead: Is There a Corneal Sepsis?

Microbial, infectious keratitis is a relevant indication for penetrating keratoplasty. The requirement for transplantation results in histopathological examination of the entire thickness of the cornea. Although the clinical diagnosis is not always possible to confirm, pathology can support diagnostic evidence of clinical presentation and pathogenesis. This is achieved with multiple methods from cytology, histochemistry, immunohistology, molecular pathology and in rare cases electron microscopy. These allow tissue-based detection of previous and parallel diseases and the responsible pathogens. The failure of satisfactory clinicopathological correlation raises the question whether a suspected pathogen was not ultimately responsible for destroyed corneal tissue. The pathogenesis of keratitis requiring transplantation is not yet completely understood, also on the experimental level. The development of such a keratitis can lead to a clinical symptomatology which can be described as "threatening organ dysfunction", a term used in sepsis research. Considering recent literature, possible correlations between sepsis and microbial keratitis and their relation to histopathology are discussed.

Visualization of Three-Dimentional Stigmoid Body in FFPE and Ultrathin Sections of Mouse

Huntingtin-associated protein 1 (HAP1) is abundantly expressed in the neurons of the central nervous system and forms unique intracytoplasmic inclusions of unknown function called "stigmoid bodies" (STBs). Transmission electron microscopy has revealed that the STBs are aggregates of granules containing cavities with a diameter of 0.5-3 µm. Small STBs fuse to form larger STBs, the size of which is said to vary depending on the developmental growth stage and brain region. Light microscopy can only reveal that these STBs have similar circular shapes, due to its limited resolution. Therefore, light microscopy is only fit for the study of the STB distribution and quantitative changes. We, herein, suggest the adoption of correlative light and electron microscopy, that combines confocal laser scanning microscopy and scanning electron microscopy, as the method allowing us to identify the HAP1-positive STBs in formalin-fixed paraffin-embedded (FFPE) sections. This approach allows us to study the three-dimensional morphology of immunolabeled objects in histopathological specimens. The STBs in FFPE sections of murine hypothalami reflected the transmission electron microscopic images of Epon-embedded STBs, although we were not able to observe any organelle covering the STBs of the FFPE sections. Furthermore, we were able to reconstruct the three-dimensional structure of the STB and we identified it to be of spherical form, covered with mitochondria and rough endoplasmic reticulum, and bearing a cluster of cavities in the center. In the future, we might gain new insights by comparing the 3D structure of the STB between different neurons and under a variety of conditions.

Undifferentiated Pleomorphic Sarcoma With Hyaline Globules (Thanatosomes)

Hyaline globules (HGs) or thanatosomes belong to a well-defined microscopic phenomenon common to any cell type, representing eosinophilic and round-shaped intracytoplasmic inclusions as a result of altered cellular metabolism. We experienced a case of undifferentiated pleomorphic sarcoma (UPS) of the left thigh, immunoreactive diffusely for CD99 and p16INK4a and focally for alpha-smooth muscle actin. HGs were multifocally clustered in the cytoplasm of the tumor cells. An ultrastructural study using a formalin-fixed, paraffin-embedded block was performed to visualize HGs in the UPS cells. Light microscopically, multifocally clustered HGs were PAS-positive with diastase-resistance and fuchsinophilic in Masson's trichrome staining. HGs were immunoreactive for cleaved caspase-3, but negative for ubiquitin. Ultrastructurally, apoptotic tumor cells contained clusters of small-sized electron-dense globules. Granular material was often deposited in the globule matrix. The formation of the HGs is supposedly related to an apoptotic process of the tumor cells. Though a nonspecific and minor microscopic finding, HGs in soft tissue sarcomas may represent a useful histologic marker of enhanced cell turnover and/or ischemic injury. This is the third report describing HGs in UPS.

Pancreatic Intraductal Papillary Mucinous Neoplasm with Hyaline Globules (Thanatosomes): Report of Two Cases

Hyaline globules (HGs; thanatosomes) represent a morphologic entity representing a metabolic imbalance common to all cell types. HGs, intracytoplasmic eosinophilic globular accumulations of proteinaceous material of varying sizes, have been observed in varied tumors and benign tissues. Different explanations have been proposed for their formation, according to the tumor type and anatomic location. An earlier study suggested that HGs were closely related to apoptosis. There are some reports describing HGs in pancreatic neoplasms, such as intraductal oncocytic papillary neoplasm, solid-pseudopapillary neoplasm, oncocytic endocrine neoplasm, and invasive ductal adenocarcinoma; however, this is the first report describing HGs in pancreatic intraductal papillary mucinous neoplasm (IPMN). An ultrastructural study was performed to visualize HGs in two pancreatic IPMNs of gastric type (one non-invasive malignancy and another adenoma). Light microscopically, intracytoplasmic HGs were clustered multifocally. HGs were periodic acid-Schiff-positive and diastase-resistant, and fuchsinophilic with Masson’s trichrome stain. The diameter ranged from 4.7 to 20.6 μm (mean: 13.3, median: 14.1). They were mainly seen at the supranuclear position and occasionally with subnuclear location. Ultrastructurally, HGs were round in shape and homogenously electron-dense without mitochondria or chromatin-like condensation. The nuclei of HGs-containing mucous columnar cells appeared intact without evidence of apoptosis. It is worth emphasizing that HGs in the pancreatic IPMN of gastric type belong not to apoptotic bodies but to proteinaceous secretory materials.

Colitis nucleomigrans: The third type of microscopic colitis (part 2). An ultrastructural study

In the preceding article (part 1), we proposed the third type of microscopic colitis: colitis nucleomigrans (CN). Microscopically, the nuclei of surface‐lining columnar cells were migrated in chain to the middle part of the cells, and apoptotic nuclear debris was scattered in the cytoplasm beneath the nuclei. For ultrastructural analysis, buffered formalin‐fixed biopsy tissue of CN (n = 2) was dug out of paraffin blocks. After deparaffinization, tissue blocks were prepared with conventional sequences. Ultrathin sections were stained with uranyl acetate and lead citrate. Fine morphological preservation was satisfactory even after paraffin embedding. Apoptotic nuclear debris was localized within the cytoplasm beneath the migrated nuclei of the surface‐lining columnar cells. Abnormality of cytoskeletal filaments (actin, cytokeratin and tubulin) was scarcely recognized in the epithelial cytoplasm. Macrophages located in the uppermost part of the lamina propria phagocytized electron‐dense globular materials. Intraepithelial lymphocytes with scattered dense bodies were observed among the columnar cells. We suppose that altered apoptotic processes in the colorectal surface‐lining epithelial cells may be involved in the pathogenesis of CN. Mechanisms of nuclear migration to the unusual position or impairment of nuclear anchoring to the basal situation in the surface‐lining epithelial cells remain unsettled, because cytoskeletal components showed little ultrastructural abnormality.

Adenocarcinoma with Neuroendocrine Differentiation of the Colon Accompanying Osteoclast-Like Giant Cells

Introduction. Neuroendocrine differentiation in colorectal cancer is reportedly associated with poorer grade of tumor differentiation, nodal and distant metastasis, and other unfavorable features, contributing to a worse clinical outcome. Colorectal cancer with osteoclast-like giant cells (OGCs) is extremely rare. Case Presentation. An 86-year-old woman was diagnosed as double cancer of the transverse and sigmoid colon. Both tumors were simultaneously removed. The transverse colon cancer directly invaded the area of the right gastroepiploic vessels and spread to the nodes and histologically consisted of both the tubuloglandular and solid components. CD8/granzyme B-positive tumor-infiltrating lymphocytes and CD163/CD68-positive macrophages, frequently forming OGCs, were observed particularly at the invasion front. The carcinoma cells were labeled focally for synaptophysin and diffusely for the DR locus of the human leukocyte antigen and programmed death-ligand 1 (PD-L1). Deficient expression of DNA mismatch repair (dMMR) proteins was immunohistochemically confirmed. The patient died 16 months after surgery. Conclusion. This is the first report of colonic adenocarcinoma with neuroendocrine differentiation accompanying OGCs. Histopathologic factors of the poor prognosis in the present case included (a) the presence of more than 2% cells with neuroendocrine differentiation, (b) infiltration of CD163/CD68-positive OGCs at the invasion front, (c) deficiency of dMMR proteins, and (d) PD-L1 expression.

The NanoSuit method: a novel histological approach for examining paraffin sections in a nondestructive manner by correlative light and electron microscopy

Histological examination using the light microscopy is currently the gold standard for life science research and diagnostics. However, magnified observations are limited because of the limitations intrinsic to light microscopy. Thus, a dual approach, known as correlative light and electron microscopy (CLEM), has emerged, although several technical challenges remain in terms of observing myriad stored paraffin sections. Previously, we developed the NanoSuit method, which enabled us to keep multicellular organisms alive/wet in the high vacuum of a scanning electron microscope by encasing the sample in a thin, vacuum-proof membrane. The approach uses the native extracellular substance (ECS) or an ECS-mimicking substance to polymerize a membrane by plasma or electron beam irradiation. Since the resulting NanoSuit is flexible and dense enough to prevent a living organism's bodily gas and liquids from evaporating (which we refer to as the "surface shield enhancer" (SSE) effect), it works like a miniature spacesuit with sufficient electron conductivity for an SEM observation. Here, we apply the NanoSuit method to CLEM analysis of paraffin sections. Accordingly, the NanoSuit method permits the study of paraffin sections using CLEM at low and high magnification, with the following features: (i) the integrity of the glass slide is maintained, (ii) three-dimensional microstructures of tissue and pathogens are visualized, (iii) nuclei and 3,3'-diaminobenzidine-stained areas are distinct because of gold chloride usage, (iv) immunohistochemical staining is quantitative, and (v) contained elements can be analyzed. Removal of the SSE solution after observation is a further advantage, as this allows slides to be restained and stored. Thus, the NanoSuit method represents a novel approach that will advance the field of histology.

Microsporidial keratitis retrospectively diagnosed by ultrastructural study of formalin-fixed paraffin-embedded corneal tissue: a case report

Background

The utility of formalin-fixed paraffin-embedded (FFPE) corneal tissue specimens for retrospective diagnosis of microsporidial keratitis was evaluated by transmission electron microscopy (TEM) analysis and the possible second case of microsporidial keratitis after Descemet stripping automated endothelial keratoplasty (DSAEK) was described.

Case presentation

A 68-year-old man presented with multiple crystalline opacities in the corneal stroma that progressed extremely slowly after DSAEK. Fungiflora Y staining of corneal scrapings from the affected regions revealed an oval microorganism. Topical voriconazole administration was ineffective and penetrating keratoplasty was performed. Histological and molecular analyses were carried out on the excised cornea. Ziehl–Neelsen staining revealed an acid-fast, oval organism that was visible by ultraviolet illumination after Fungiflora Y and Uvitex 2B staining, whereas periodic acid-Schiff and Grocott’s staining did not yield any significant findings. Microsporidium was detected by TEM of FFPE tissue. Nosema or Vittaforma sp. was suspected as the causative microorganism by PCR of FFPE tissue and by the fact that those species are known to cause eye infection. The corneal graft has maintained transparency at 1 year and half postoperatively.

Conclusions

This is the first known case of microsporidial keratitis diagnosed retrospectively by molecular and ultrastructural study of FFPE tissue, and the possible second case of microsporidial keratitis after DSAEK. Microsporidial keratitis should be considered when corneal opacity refractory to conventionally known therapy would occur after DSAEK. Our findings suggest that more microsporidial keratitis cases than have been reported to date can be identified by TEM or PCR examination of FFPE corneal specimens.

Hypertension-induced changes in the rat myocardium during the development of cardiac hypertrophy - a comparison between the left and the right ventricle

The hypertrophy of the cardiac muscle is one of the most significant maladaptive mechanisms activated in response to increased workload. It is associated with histological and ultrastructural alterations, changes in the quantitative parameters and the expression of different enzymes. While the structural and functional consequences of systemic hypertension on the left ventricle have been well evaluated, the right ventricle has received less attention. The aim of the present study was to analyse and compare the changes in the left and right ventricle during the development of cardiac hypertrophy initiated by systemic hypertension in different age groups of spontaneously hypertensive rats. Therefore, we studied the histology and ultrastructure of the cells of the myocardium, evaluated the immunohistochemical expression of the enzyme neuronal nitric oxide synthase and conducted a quantitative analysis of several morphometric parameters. We used three groups of spontaneously hypertensive rats. For the quantitative analysis we also used three age groups of age- and weight-matched control animals (normotensive Wistar rats). In both ventricles, we described cardiomyocytic hypertrophy, focal myocytolysis and increased collagen deposition in the interstitial space. Our observations on the ultrastructural level were associated with changes in the cardiomyocytic nuclei, the arrangement, maturity and organisation of the myofibrils, the localisation and ultrastructure of the mitochondria, the development and maturity of the intercalated discs, as well as changes in the components of the interstitium. The immunohistochemical expression of neuronal nitric oxide synthase in the left ventricle was stronger than that in the right ventricle across all age groups. The comparative quantitative analysis revealed that changes in the studied morphometric parameters in the two ventricles occurred disproportionately. In conclusion, the present study characterised the development of cardiac hypertrophy in response to systemic hypertension in both ventricles and demonstrated the involvement of the right ventricle.