PARAFFIN SECTIONS 



260 



PARAFFIN SECTIONS 



purposes a paraffin with melting point 

 of 56-58°C. is employed; but 6(>-62°C. 

 is sometimes selected for very thin sec- 

 tions and 52-54''C. for thick ones. 

 Paraffins of low melting points are de- 

 scribed by Waterman, H. C, Stain 

 Techn., 1939, 14, 55-62. When it is 

 desired to give the imbedding medium 

 more firmness than 60-62 °C. paraffin, 

 use is occasionally made of Rubber 

 Paraffins or Ceresin. Under Clarite 

 is described a mixture of paraffin and 

 clarite for use in hot weather when thin 

 sections are demanded. Routine paraf- 

 fin infiltration is best done in wide 

 mouthed glass bottles or jars in an in- 

 cubator held at the proper temperature. 

 Excessive temperatures harden and 

 shrink the tissues. The paraffin over 

 each specimen should be changed at 

 least once to insure removal of the xylol 

 or other clearing agent. If this removal 

 is incomplete difficulties will be later 

 encountered in crystallization of the 

 paraffin block and in sectioning. The 

 time necessary for infiltration will de- 

 pend on the size of the tissue and its 

 penetrability. Five to 6 hours is about 

 the average with limits of 2 to 24 hours 

 in special cases. See special treatment 

 for Teeth and Bone. 



For actual imbedding, folded paper 

 containers have now been rather gen- 

 erally replaced by glass dishes. Watch 

 glasses (Syracuse preferred) are satis- 

 factory ; but Petri dishes, the inner sides 

 of which are not quite vertical but slope 

 outward slightly from the base, are bet- 

 ter. First smear a little glycerin evenly 

 over the bottom and sides of the dish. 

 Then pour in a little paraffin, a thin 

 layer of which will harden so that when 

 the tissue is placed in the dish, it will 

 not come in contact with the bottom. 

 It is customary to orient the tissue so 

 that the surface to be cut first is next 

 to the bottom of the dish. Quickly 

 pour in more paraffin until the tissue is 

 covered to a depth of say 6 mm. Hold 

 the dish in ice water until the surface 

 of the paraffin has hardened just to the 

 point when on immersion in the iced 

 water the surface will hold its shape and 

 not run. However too rapid cooling 

 of paraffins of high melting point may 

 cause cracks in the surface and even in 

 the depth of the blocks. After a few 

 minutes the paraffin block slips out 

 easily because the glycerin prevented it 

 from sticking. When several different 

 specimens are imbedded in the same 

 dish identify each by partly imbedding 

 near it a small strip of paper bearing its 

 number. Finally some of the paraffin 

 is cut away from each tissue so that it 

 can be conveniently filed away but it is 



important not to remove too much 

 paraffin . 

 Paraffin Sections. 1. Blocking. If the 

 specimen is a slice of tissue it was 

 trimmed at the time of fixation into a 

 quadrangular form with each edge and 

 surface parallel to the opposite one. If 

 the specimen is a cross section of a 

 tubular structure the cutting will be 

 more difficult. Heat the metal holder 

 of the microtome, gently press the sur- 

 face of the paraffin block against it and 

 harden in iced water. The surface of 

 tissue, protected by the most paraffin 

 (which is the upper surface, remote 

 from bottom of the dish, as it was im- 

 bedded), should be next to the holder 

 and as far as possible evenly equidistant 

 from the surface of the holder. Unless 

 there is plenty of paraffin between the 

 tissue and the holder, difficulties will 

 be encountered if it becomes necessary 

 to remount the block on subsequent oc- 

 casions to cut more sections. Since the 

 slice of tissue is of even thickness its 

 outer surface will be evenly parallel 

 to the sweep of the knife so that the 

 tissue included in a given section will 

 be approximately the same distance 

 from the surface of the block and equally 

 subjected to fixation and subsequent 

 technique. 



2. Cutting. The knife should cut 

 from long side to opposite long side. 

 Trim the edges of the paraffin block so 

 that it will have to pass through an even 

 layer of paraffin at least 5 mm. wide both 

 before and after it enters and leaves the 

 tissue. When more paraffin is cut away 

 it may be later needed if more trimming 

 is required to make the sections into 

 straight ribbons. The sides of the 

 tissue should also be protected by layers 

 of paraffin which are parallel and of even 

 thickness. The object of all this is for 

 the knife to cut through the paraffin and 

 tissue squarely and for it to encounter 

 as nearly as possible equal resistance. 

 The resistance of the paraffin at the sides 

 will, however, always be less than that 

 of the paraffin plus the tissue at the cen- 

 ter. For this reason it may be necessary 

 to cut away most of the paraffin from the 

 sides. 



But all specimens are not rectangular 

 slices of tissue of uniform thickness. 

 Spherical bodies are easy to cut but the 

 sections obtained are very difficult to 

 flatten. Specimens containing large 

 cavities are troublesome because the 

 paraffin in the cavities offers so little 

 resistance. In such cases celloidin im- 

 bedding is advised. When a part of the 

 tissue is brittle and the rest soft it is 

 best to orient the tissue so that the knife 

 passes through the soft part first. In 



