Histotechnology I- Miscellaneous
Question No. 18. The Answer is: (B) The specimen has been fixed in formalin and then frozen (freezing artifact).
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Discussion: "Swiss cheese" artifacts can be generated by post mortem over growth of gas producing bacteria. The size can vary from millimeters to over 1 centimeters. Just like the vacuoles in Swiss cheese, their distribution is irregular and there is a significant variation in size. In contrast to the vacuoles being shown here, the vacuoles in "Swiss cheese" artifact have smooth margins. Although enzyme digestion can be used to retrieve signal in immunohistochemistry and in situ hybridization, it is not a required step for modified Bielschowsky stain which is a silver stain that is often used in the study of neuropathology. In addition, the holes that are created by over digestion is never so sharply defined. Wax cracks typically appear as irregular, elongated slits in the tissue section. Knife marks are straight, very long and often appear as parallel lines. These features are not present here.
In the section being shown, there is a randomly regular distribution of vacuoles of comparable size in the grey matter. The extent of involvement is similar in different parts of the photo. There is also similar involvement in the white matter albeit the the vacuoles are far smaller. This difference is well appreciated in the medium magnification photo. This type of artifact is produced by freezing tissue that has been fixed in formalin. This brain has been fixed in formalin and solidly frozen before paraffin sections are made. This type of artifact can be avoided by cryoprotection with concentrated sucrose solution if frozen sections are to be done on formalin fixed tissue blocks.