For semi-thin sections, the tissue is not embedded in paraffin (as is otherwise common for light microscopy), but in acrylic or epoxy resins, which are harder embedding substances, and permit thinner slices, so-called semi-thin slices of about 0.5 max. 2µm in thickness, to be produced. After embedding in paraffin, a section is > 3µm thick. Semi-thin sections have better resolution under a light microscope than the thicker paraffin sections.
Embedding in epoxy resins is always performed when tissues are to be examined under a transmission electron microscope (TEM). Semi-thin sections are used to study the specimen under the light microscope before being examined under the transmission electron microscope (TEM).
Toluidine blue is primarily used to stain semi-thin sections since most dyes do not sufficiently infiltrate the epoxy resin. Toluidine blue stains various biologic components to different shades of blue.