ATOMIC-FORCE AND SCANNING ELECTRON-MICROSCOPY OF XENOPUS-LAEVIS OOCYTES

The atomic force microscope (AFM) is increasingly used to image biolog ical objects such as DNA, actin, membranes, and cells, in air or in li quid. It is very important that certain biological objects are observe d in buffer, their ''natural'' environment. Xenopus oocytes form a goo d model system for expressing foreign genes, since their size easily a llows the introduction of cDNA, coding for membrane proteins. Since oo cytes are large objects, they have to be mounted so as to be immobiliz ed during scanning by the AFM tip. We have used different supports for mounting Xenopus oocytes: pipettes, embedding in plexiglas, and gelat ine. We have attempted to image the cell membrane of the oocyte with t he AFM, but could only visualize the vitelline envelope and the yolk p latelets. In the scanning electron microscope, we found that the cell membrane of the oocyte is rich in microvilli. The difficulty in imagin g oocyte membrane with an AFM comes from the fact that the microvilli make the cell membrane of the oocyte very rough and deformable, thus c reating instabilities for the tip during the imaging process. It thus seems impossible to image proteins that are inserted in the cell membr ane of oocytes bearing microvilli. The use of oocytes with practically no microvilli may perhaps solve the problem of instability.