The sections for examination under the transmission electron microscope are about 0.1µm thick. They are referred to as ultra-thin sections.
To obtain such ultra-thin sections, the tissues are embedded in plastic polymers like epon (instead of paraffin, which is used for examination under the light microscope) after fixation and dehydration. Ultra-thin sections are not stained with dyes, but contrasted with heavy-metal salts. The heavy-metal salts lead to a different electron scatter and thereby create a differentiated blackening of the photographic negative.
A common method of creating contrast results with 5% uranyl acetate and lead citrate.