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D. E. Bradley 



CELL ULTRASTRUCTURE IN MAMMALS 



The term "ultrastructure" refers to the 

 fact that the cell structures accounted for 

 here have been analyzed with the aid of the 

 electron microscope. The finer details of cell 

 structures cannot be resolved with the light 

 or phase contrast microscopes and, there- 

 fore, have been considered as being beyond 

 or ultra this level of resolution. 



Methods 



The techniques applied in modern electron 

 microscopy are found elsewhere (electron 

 microscopj^: specimen preparations). It 

 should be emphasized, however, that thin 

 sectioning (section thickness about lOOA) 

 is required to permit the best resolution. 

 Several specially designed microtomes are 

 commercially available such as the Porter- 

 Blum (U. S. A.), the Sjostrand (LKB, 

 Sweden), the Sitte (Reichert, Austria), the 

 Moran (Leitz, Germany), the Hanstra 

 (Philips, Holland). The microtome preferred 

 by the author is the LKB Ultrotome (Swe- 

 den) which is the most recent and most 

 superior in design. 



The preservation of the tissue has been 

 thoroughly worked out by Dr. Palade 

 (U. S. A.) and Dr. Sjostrand (Sweden). It is 

 believed that any analysis of the cell ultra- 

 structure today can be made without the risk 

 of describing artifacts because so much is 

 known about how various factors may influ- 

 ence the cell structures: the tonicity and 

 acidity of the fixative, the temperature, post 

 mortem changes, etc. 



The contrast of the electron microscopic 

 picture (electron micrograph) can be en- 

 hanced by applying special staining tech- 

 niques, either during the fixation or dehy- 

 dration periods or after sectioning. The 

 fixative itself gives, however, cjuite sufficient 

 contrast for most studies. The most com- 

 monly used is osmium tetroxide (often called 

 osmic acid, although it is not an acid). After 

 fixation, the specimen is dehydrated in 

 graded alcohols and subsequently embedded 

 in liquid plastics (usually a mixture of butyl 

 and methyl methacrylates). The monomers 

 are polymerized and the embedded tissue 

 block is then ready to be sectioned. 



Cell Shape 



The general shape of cells varies from tis- 

 sue to tissue (Fig. 1). This has been ac- 

 curately analyzed with the light microscope 

 and very little has been added to previous 



91 



