transmission electron microscopy (TEM)
/ tranz-MISH-un ee-LEK-tron my-KROS-koh-pee /
Think of holding a thin slice of stained jelly up to a bright lamp: the dark bits block the light and the clear bits let it through, so you see a shadow picture of what is inside. Transmission electron microscopy does exactly this, but with electrons instead of lamplight. A beam of electrons is sent straight through an extremely thin slice of cell, and the parts that block electrons appear dark, mapping out the cell's internal structure in stunning detail.
In a TEM, the specimen must be sliced incredibly thin — often less than a tenth of a micrometre — so that electrons can pass through it. Heavy-metal stains such as lead or uranium salts are soaked into the sample because they scatter electrons strongly; dense, metal-rich regions block the beam and show up dark, while regions that let electrons pass appear bright. The transmitted electrons strike a detector or screen below, building up a flat, two-dimensional cross-section with resolution down to fractions of a nanometre.
TEM is the workhorse for seeing the inside of cells at the highest detail: the double membrane of the nuclear envelope, ribosomes studding the endoplasmic reticulum, the layered cristae of mitochondria. Its great limitation is that it shows only thin, dead, chemically processed slices — never a whole living cell, and never true colour (the colours in published images are added afterward). Because you see one thin slice at a time, reconstructing the full three-dimensional shape takes many slices and careful interpretation.
A TEM image of a thin slice through a cell shows the nuclear envelope as two parallel dark lines with tiny gaps — the nuclear pores — strung along it like beads.
TEM excels at internal cross-sections but shows only one ultra-thin, dead slice at a time.
TEM produces flat, internal slices, not surface views; the vivid colours in textbook electron micrographs are added by computer, since electrons carry no colour information.