Visualization and rapid quantification of autoradiographic labeling in scanning electron microscopy applied to localization of receptor sites on the surface of whole cells.

The use of backscattered electron imaging (BEI) as a routine procedure for examining autoradiographic reactions in scanning electron microscopy (SEM) is described. This technique allows the determination of the number of receptor sites occupied by 125I-epidermal growth factor (EGF) on whole cells. The effect of 1.25 dihydroxyvitamin D3 (1,25 (OH)2D3) on the number of epidermal growth factor receptors (EGF-R) in the BT 20 human mammary carcinoma cell line (which is known to possess a very high number of EGF-R) has been evaluated with this method. To compare the silver grain density over the cells (controls and 1,25 (OH)2D3-treated cells) we used an image analysis system Quantimet 900. The results were compared with those of a previous study using transmission electron microscopy (TEM). This study confirmed the results obtained with TEM and showed the even distribution of receptors sites on a single cell and a large difference in the number of receptor sites from one cell to another. The use of BEI to visualize the autoradiographic reaction in SEM allowed the examination of a large surface with good contrast and resolution and eliminated artefacts not corresponding to the silver grains. It gave new information not delivered by quantitative TEM autoradiography and was easier and faster to use. The efficient use of SEM autoradiography combined with BEI could facilitate whole area distribution mapping of radioactive labeling.