In pathology-speak, Electron Miscroscopy is often referred to as "EM" or "ultrastructural examination."

EM is time consuming and technically challenging, but allows the visualization of structures much, much smaller than anything visible on light miscoscopy.  EM allows visualization of mitochondria and other organelles, the neuromuscular junction, and the structure ("ultrastructure") of things like the cytoskeletal architecture and sarcomere.

On EM, things which are dark are "osmiophilic," since osmium is used as a fixative.