English: A sample of cells embedded in epoxy resin (amber) for transmission electron microscopy (TEM). The block of epoxy resin is mounted in an ultramicrotome sample. Cells have to be embedded in a hard substance such as epoxy resin for electron microscopy as very thin sections (typically 70 to 350 nm thick) have to be cut for imaging. This block has been trimmed ready for sectioning, the sections will be cut from the 1x1 mm flat square in the center. The cells (black, center) have been fixed with glutaraldehyde and osmium tetroxide then stained with uranyl acetate and are therefore now dark in colour.