Processing light microscopy sections for electron microscopy
 Z. Kristóf, O. Tímár, K.Imre
 Department of Plant Anatomy, Eötvös L. University, Pázmány s.1/C 1117 Budapest,  Hungary.

 Because of the different section thickness a gap exist between light and electron microscopy. It is almost impossible to view light microscopy sections in conventional transmission electron microscope and vice versa. An old requirement is to study same details first with light microscope and then with electron microscope. 
A useful  solution for this problem is to make light-microscopy sections which can then be re-embedded and subjected to ultrathin resectioning. The most common method is to invert a resin-containing capsule on to the section still on the glass slide and cure in situ. The main drawback of these procedures is the long processing times due to the slow curing of resin.
 In the present study, we report our results with a technique developed for processing the light microscopy sections for ultrathin resectioning in such a way that the processing time is decreased from one day to a few minutes [1]. This method is based on sticking the section to a resin stub with a light curable dental bond instead of the time-consuming re-embedding. The procedure was used in the electron microscopic study on the egg apparatus of Torenia.
 After fixation and dehydration the ovules of Torenia fournieri were embedded in Spurr resin. 5*m thick serial sections were collected and dried onto glass slides.
Unstained and stained (Antimonate, Acridin Orange) sections were mounted in water and covered with a cover glass for light microscopy. Images were grabbed through a video camera  attached to the microscope. These images were printed with a standard laser printer to help the orientation of the ultrathin section in the electron microscope.
Alternatively, when no detailed light microscopic investigations or good quality images were needed, the selection was carried out directly in the inverse microscope
The selected, dried section was set in the centre of the viewing field in the inverse microscope at low magnification. An empty resin block surface was slightly wetted with dental bond. (ERR CSR Bond, item No. 20730).The block was placed onto the selected section and positioned precisely. The whole positioning  was carried out on the stage of the inverse microscope. (fig. 1.)
After positioning the blue light was let through the objective. After two minutes exposure the curing was complete and the resin block was attached to the slide. To remove the block and detach the section from the microscope slide a rapid cooling of the slide with liquid N2 was applied and only a gentle force was needed to detach the section. The block was immediately set in the ultramicrotome and resectioned. 30-70nm thick  serial sections were cut with a diamond knife without problem for conventional electron microscopy and for elemental analysis  and mapping.(fig.2.)

References

1.Kristóf Z. Journal of Microscopy 188(1997) 88.