STRUCTURE AND ACTIVITIES OF CELLS 



a light microscope, a stream of electrons from a heated tungsten filament is 

 directed, in a vacuum, through an extremely thin slice of tissue onto a photo- 

 graphic plate where the magnified image of the cell can be recorded. The 

 preparation of tissue slices no more than 0.1// in thickness requires great skill 

 in microtechnique; sections for study with the light microscope range from 

 3 to S/j. in thickness. Prints of the photographic negative of the image are 

 usually enlarged so that cellular structures magnified hundreds of thousands 

 of diameters are available for study in an electron micrograph. 



The techniques discussed so far made possible the accumulation of informa- 

 tion about the parts of cells as they exist together in their normal relation- 

 ships. Additional knowledge of cells has come from separating these parts in 

 order to establish their integrity and to seek to determine their functions. 

 In the technique of microdissection, very fine glass needles are mechanically 

 manipulated to separate parts of cells with great precision under the light 

 microscope; movements of the delicate instruments can be controlled over a 

 distance as short as 0.5 m^i. With micropipettes, parts can be sucked out of 

 one cell and transferred to another. The more drastic techniques of cell 

 fractionation involve grinding or homogenizing the cells and then centrifuging 

 the resultant conglomerate. Particles of different densities are aggregated in 



Fig. 2.3. Diagrams of light and electron 

 microscopes to suggest similarity. The 

 lenses in the light microscope are glass. 

 In the electron microscope, the lenses are 

 magnetic fields, the strength of which can 

 be varied in order to focus the image on 

 the viewing screen. This can be turned 

 aside to permit recording of the image on 

 the photographic plate. (Courtesy Radio 

 Corporation of America.) 



A 



Light 



microscope 



B 



Electron 

 microscope 



Lamp 



Tungsten i i 

 filament v/ 



Condenser lens 



Specimen 

 Objective lens 



Ocular Projector 

 lens lens 



Human eye 



Image 



Viewing screen 

 (photographic plate) 



15 



