Ultra thin cut
The ultra thin section is a method used - among other things in biology - to prepare objects for viewing with a transmission electron microscope .
Procedure
Before a biological object can be viewed in the transmission electron microscope, it has to be prepared in a process that takes several days .
First, the biological material is converted into various substances ( osmium tetroxide , formaldehyde or glutaraldehyde ). The cells die in the process, but the natural structures remain largely unchanged. After this chemical fixation , the specimen is dehydrated, embedded in synthetic resin and cut with an ultramicrotome . The ultramicrotome's diamond knife enables layers to be cut off 50 nanometers thick . One page of a book is about 1000 times thicker than such a preparation. When cutting, the specimen is caught on a drop of water and fished from there with a fine metal net. The object can be introduced into the beam path of the electron microscope on this carrier network.
If overview images are desired, pinhole diaphragms with a large opening of 1 or 2 mm in diameter should be used instead of carrier nets. These are first coated with a Formvar film made from the liquid phase (thickness = 2-3 nanometers). After it has been stabilized and checked for quality in the electron microscope, the ultra-thin section is applied so that it appears like a flying carpet in a complete picture (see photo).
While with the light microscope the different light transmittance of the structures causes image contrasts , with the electron microscope there are differences in the electron transmittance . However, these are very low in biological structures. Therefore the preparation has to be contrasted . This is already possible when fixing with osmium tetroxide . Preparations from the field of palaeobotany do not require this contrasting. Such objects are more or less strongly contrasted in the sediment by phases of coalification .