MICKOTOMY 



MICROTOMY 



Thanks to the contributions of the Ger- 

 man physicist Abbe the efficiency of the 

 hght microscope was rapidfy improved in 

 the latter half of the 19th century. This 

 development in turn implied that completely 

 different demands had to be made on prepa- 

 ration of specimens. In order to make use 

 of the increased resolution power of the mi- 

 croscope, it was necessary to employ thin 

 sections in the study of biological material. 

 In this way the development of microtomy 

 was started. This development was both a 

 conseqvience and a necessary precondition of 

 modern light microscopy. The limit for the 

 most important property of the microscope, 

 the resolving power, is determined by the 

 wave length of the light employed. As far as 

 the light microscope is concerned, we can 

 therefore only expect technical improve- 

 ments in the future. Regarding the technique 

 for making microscopical preparations, it 

 seems however justified to hope for consider- 

 able progress. 



Today the routine work in human pathol- 

 ogy constitutes the overwhelming part of 

 light -microscopical work on biological mate- 

 rial. The cutting technique for this purpose 

 is well developed, and generally known, and 

 further information is therefore not needed. 

 Contrary to this, severe problems are en- 

 countered, in quantitative morphological 

 and c>4ochemical work. Like the other steps 

 in the preparation of sections, cutting dis- 

 torts the native tissue. If this is not taken 

 into consideration, errors may arise in quan- 

 titative work. The aim of the present article 

 is to illustrate these problems. 



The Microtome. When classical his- 

 tology was developed, the demands with 

 respect to precision which were made on a 

 microtome corresponded to the best which 

 could be produced. Today the situation is 

 different. The progress of modern precision 

 industry makes the production of good mi- 

 crotomes an easy task. Only in the produc- 

 tion of ultramicrotomes for cutting of mate- 



rial for electron microscopy are modern 

 resources fully exploited. In principle a 

 microtome produces two different move- 

 ments between the knife and the prepara- 

 tion: (1) the cutting movement occurring in 

 the cutting plane, and (2) the feeding move- 

 ment perpendicular to this plane. In a mi- 

 crotome of good quality both these move- 

 ents must be as precise as possible. The 

 true feeding should not deviate more than 

 0.1 M from the microtome setting, when the 

 latter is in the range 1-20 m- Measurements 

 on various slide and rotation microtomes 

 show that this demand is met by most 

 microtomes from well known firms. It is 

 thus not the microtome which causes the 

 problems in the cutting for quantitative 

 analysis. 



The Knife. Besides a good microtome it 

 is also necessary to have a knife with optimal 

 qualities. Ever since the cutting of biologi- 

 cal material began, much work has been 

 devoted to getting as good knives as pos- 

 sible. A large selection of suitable kinds of 

 steel is a^^ailable, varying from pure carbon 

 steel to various types of multialloyed and 

 specially hardened stainless steel. Thi-ee 

 qualities of the steel are of special impor- 

 tance (1) the hardness, (2) the toughness 

 and (3) the corrosion resistance. Unfor- 

 tunately these parameters do not vary in a 

 parallel manner. A very hard steel is brittle 

 and increased addition of stainless metals 

 reduces the hardness. Knife steel therefore 

 represents a compromise which, however, 

 thanks to modern metallurgy meets exacting 

 demands. 



The edge of the knife is the section line 

 between the two facet surfaces. In order to 

 understand which qualities the edge must 

 possess it is necessary to illustrate schemati- 

 cally how cutting is performed (Fig. 1). 

 During cutting, a distortion of the tissue 

 occurs, causing the thickness of the section 

 ti to be larger than the feeding of the block 

 /i . For the cutting three angles are of im- 

 portance, (1) the rake angle (r), (2) the 

 bevel angle (b), and (3) the clearance angle 



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