*2.7. TEM studies of Cell Cultures and Spheroids*

All reagents for microscopic studies were purchased from EMS (Hatfield, PA, USA).

Samples of fixed cell cultures and spheroids were washed from paraformaldehyde with Hank's balanced solution and were postfixed with 1% osmium tetraoxide solution for 1 h, dehydrated in ethanol and acetone according to the standard method, and then embedded in an epon-araldite mixture to obtain hard blocks.

Ultrathin and semithin sections were prepared on an ultramicrotome EM UC7 (Leica, Wetzlar, Germany) using a diamond knife (Diatome, Nidau, Switzerland). The semithin sections of spheroids were stained with Azur II and were examined in a Leica DM 2500 light microscope (Leica, Wetzlar, Germany) to choose an area for ultrathin sectioning. Ultrathin sections were contrasted with 2% water solutions of uranyl acetate and lead citrate and examined in a JEM 1400 TEM (JEOL, Japan). Digital images were collected using a Veleta side-mounted camera (EM SIS, Muenster, Germany).
