Electron Microscopy
Electron microscopy uses a beam of electrons to image tissue at a magnification of up to several hundred thousand times. Transmission electron microscopy, the diagnostic type, shows cell organelles, membranes, deposits and microorganisms that cannot be resolved with the light microscope.
Tissue is fixed in glutaraldehyde, post-fixed in osmium tetroxide, embedded in resin, cut into ultrathin sections (70 to 90 nm) and stained with heavy metals. The sections are examined on a transmission electron microscope, and images are recorded digitally.
It has a small but important role in diagnosis. It is the key test in a renal biopsy when light microscopy and immunofluorescence do not give a final answer, and it is needed for primary ciliary dyskinesia, some storage diseases, mitochondrial disease and the classification of a few tumours.
The test needs special fixation, equipment and expertise, so it is done in few laboratories. The result takes 1 to 4 weeks. Only a small amount of tissue is examined, which creates a sampling problem.
Indications.
- Renal biopsy: nephrotic syndrome in children and adults, haematuria with proteinuria, suspected Alport syndrome and thin basement membrane disease, membranous nephropathy (stage), fibrillary and immunotactoid glomerulonephritis, dense deposit disease, amyloid versus other organised deposits, and transplant glomerulopathy.
- Primary ciliary dyskinesia: ultrastructure of cilia in nasal or bronchial brushings or biopsy.
- Storage diseases: lysosomal (Fabry, Gaucher, Niemann-Pick), glycogen storage, and mitochondrial disease.
- Muscle and nerve biopsy: inclusions, mitochondrial changes and myelin abnormalities.
- Tumour diagnosis when immunohistochemistry is not decisive: neuroendocrine granules, melanosomes, Birbeck granules (Langerhans cell histiocytosis), and lineage in poorly differentiated tumours.
- Infection: viruses in tissue and stool (diarrhoea, gastroenteritis), and organisms such as Whipple bacilli and microsporidia.
- Platelet and sperm ultrastructure, and inherited disease of platelets.
Not indicated when light microscopy, immunofluorescence or immunohistochemistry will answer the question.
Specimen requirements.
- Small pieces (1 mm cubes) of fresh tissue placed at once in cold 2.5% glutaraldehyde, or in a buffered fixative chosen by the laboratory.
- Renal biopsy: a separate fragment of cortex with at least one glomerulus, set aside at the time of biopsy. If no fragment was taken, tissue can be retrieved from the paraffin block, with reduced quality.
- Formalin fixed tissue is a poor second choice.
- Ciliary studies: a fresh brushing in saline or fixative.
- Bone marrow, platelets and body fluids: pellets fixed as advised.
- Clinical information and the question to be answered.
