Electron microscopy (EM) uses a beam of electrons instead of light to image specimens, achieving resolution down to 0.1 nm — approximately 1000 times better than light microscopy. While largely replaced by immunohistochemistry and molecular techniques for many diagnoses, EM remains essential for specific clinical questions in renal, neuromuscular, and ciliary pathology.
Transmission Electron Microscopy (TEM)
TEM transmits an electron beam through an ultrathin section (60-100 nm) of the specimen. Electrons interact with the specimen, and the resulting pattern is projected onto a fluorescent screen or digital detector. Dense structures (nuclei, ribosomes, glycogen) scatter more electrons and appear dark (electron-dense); less dense structures (cytoplasmic matrix) appear light (electron-lucent).
Specimen preparation for TEM is demanding. Tissue is fixed in glutaraldehyde (2.5% in cacodylate buffer) which preserves ultrastructure better than formalin. Post-fixation in osmium tetroxide stabilizes lipids. Samples are dehydrated through graded ethanols, infiltrated with epoxy resin, and polymerized at 60°C for 24-48 hours. Ultrathin sections are cut on an ultramicrotome using a diamond knife, collected on copper grids, and stained with heavy metals (uranyl acetate, lead citrate) to enhance contrast.
Scanning Electron Microscopy (SEM)
SEM scans an electron beam across the specimen surface, detecting secondary electrons emitted from the surface atoms. This produces three-dimensional topographical images with resolution of 1-10 nm. SEM is used to examine surface architecture of tissues — cilia, microvilli, endothelial surface, and foreign materials. Specimens require critical-point drying and sputter-coating with gold or platinum to conduct electrons.
Diagnostic Applications of TEM
Renal pathology remains the most important diagnostic application of TEM. Glomerular diseases are classified by ultrastructural findings: electron-dense deposits in lupus nephritis (mesangial, subendothelial, subepithelial), thin basement membrane nephropathy (basement membrane thickness <200 nm), Alport syndrome (irregular thinning, thickening, and lamellation of the glomerular basement membrane), and foot process effacement in minimal change disease. EM is essential for classifying hereditary nephritis and is recommended as part of the standard workup for unexplained proteinuria and hematuria.
Neuromuscular pathology — EM identifies structural abnormalities in muscle and nerve biopsies. Specific findings include nemaline rods (congenital myopathy), central core disease (core structures lacking mitochondria), tubular aggregates, and mitochondrial crystalline inclusions. In peripheral nerve, EM detects unmyelinated nerve fiber loss, axonal degeneration, and demyelination patterns.
Ciliary pathology — EM of nasal or bronchial biopsy demonstrates the ultrastructure of cilia. Dynein arm defects, radial spoke defects, and microtubule transposition are diagnostic for primary ciliary dyskinesia (Kartagener syndrome). At least 50 cilia must be examined in cross-section for a definitive assessment.
Infectious disease — EM visualizes viral particles directly in tissue, providing diagnosis of infections where culture or PCR is negative. Characteristic viral morphology identifies herpesviruses (icosahedral nucleocapsids), adenovirus, polyomavirus (BK virus in renal transplant), and emerging pathogens.
Tumor pathology — EM resolves specific ultrastructural features: melanosomes (melanoma), Birbeck granules (Langerhans cell histiocytosis), dense-core neurosecretory granules (neuroendocrine tumors), and Weibel-Palade bodies (vascular endothelial tumors). While IHC has replaced EM for most tumor diagnoses, EM remains useful for rare undifferentiated tumors where IHC is inconclusive.
Immunoelectron Microscopy
Immunogold labeling combines IHC with EM. Antibodies conjugated to colloidal gold particles (5-20 nm) bind target antigens, visible as electron-dense dots under TEM. This technique localizes proteins to specific subcellular compartments — for example, demonstrating that a granular staining pattern on light microscopy corresponds to mitochondrial or lysosomal protein accumulation. Immunogold can be performed on ultrathin sections (post-embedding) or before resin embedding (pre-embedding).
Limitations
EM requires specialized, expensive equipment (typically $300,000-800,000 per instrument) and highly trained technical staff. Sample preparation takes 3-5 days. Sampling error is a significant risk due to the tiny area examined. Formalin-fixed tissue can be retrieved for EM but shows degraded ultrastructure compared to glutaraldehyde-fixed tissue. Many classical EM diagnoses (tumor classification) are now made by IHC, reducing the clinical volume in most laboratories to renal and neuromuscular cases.
