Question

When utilizing a transmission electron microscope, why is it necessary to stain the specimen with heavy metal salts?

Answer 1

Staining specimen with heavy metal salts (e.g. tungsten, molybdenum) allows you to see the specimen better with higher contrast when electron beam deflects off of your sample.

Answer 2

In transmission electron microscope electrons are transmitted through an ultrathin section of a specimen to form a digital image.  It necessary to stain the specimen with heavy metal salts to create a contrast such that various cell sub structures are easily visible. For example, DNA is stained with heavy salts such that it is easily visible against the background.