GENERAL MKIJOSCOrV 



Fig. 3. Apparatus for thickness measurements 

 of microtome sections. 



for quantitative work the following questions 

 arise. What are the optimal conditions for 

 sectioning? Is there any difference between 

 the thickness of the section and the feeding 

 of the microtome? How large is the thickness 

 variation within and between sections? It 

 would be an easy task to select from the 

 literature several papers, the value of which 

 may be questioned, because the authors 

 have neglected these questions. During an 

 investigation comprising more than 15,000 

 thickness determinations of sections, the 

 problem of cutting distortion has been 

 analyzed from various points of view. On 

 the basis of this investigation it is possible 

 to answer the questions stated above. 



The type of microtome employed is un- 

 important as long as it is of good equality. 

 It is likewise of minor consequence whether 

 it is driven by hand or by a motor, whether 



or not it is run continuously, and whether 

 the rate of cutting is varied within rather 

 wide limits. The type of tissue is also of little 

 importance, except of course in the case of 

 mineralized tissue. Among the embedding 

 media employed the distortion is less with 

 plastics than with paraffin. Indirect meas- 

 urements indicate that the greatest risk of 

 distortion exists with freeze sectioning. 



The properties of the knife are of decisive 

 importance for the qualities of the sections. 

 If the bevel angle is above 50°, if facet sur- 

 faces more than 10 n wide are used, or if the 

 edge line in 700 X magnification is uneven, 

 a rapid increase is observed in the ratio io/n 

 as well as in the thickness variation within 

 and between sections. If a stropped knife is 

 employed, the distortion equals that ob- 

 tained with a knife having a 70° bevel an- 

 gle. Even when the cutting conditions are 

 optimal with respect to the properties of the 

 knife, to the type of microtome, and to the 

 preparation of tissue^ — a rare event- — con- 

 siderable distortion may be anticipated. 

 First of all, an increase of the thickness 

 occiu's, which seldom is less than 10 per cent. 

 Secondly, thickness variations arise within 

 and between sections, and these are so large 

 that they cannot be neglected in quantita- 

 tive work. The coefficients for the variation 

 within and between sections in a series of 5 

 fx paraffin sections are not less than 6 and 30 

 per cent, respectively. For frozen sections 

 the corresponding value for variation be- 

 tween sections is 45 per cent. 



Duplication of Sections. A complete 

 correction for thickness variations of sections 

 would require a topographical survey of each 

 individual section. This is impossible, be- 

 cause the necessary equipment is lacking in 

 most laboratories; moreover, such a proce- 

 dure would be extremely time-consuming, 

 and the cytochemical and morphological 

 analyses of the sections seldom permit them 

 to be movmted on plane parallel slides for 

 thickness determination. Since the variation 

 between sections entirely dominates the 



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