From the Viscerotome to the Microscope
Histological techniques are operations that aim at transforming cells and tissues in preparations to be microscopically examined. These operations are developed in continued phases, according to the fundamental principles of histological techniques. In a romantic way, histological techniques are the art of staining tissues and cells, and the histotechnician is the great painter that prepares his/her paints (stains).
During yellow fever epidemics, occurred from 1930 to 1970, a fragment of the liver was removed with a viscerotome, submerged in a binder solution and sent to the viscerotomy section of the Yellow Fever Laboratory, where all kinds of histological procedures were performed for necessary diagnosis.
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| Processes for obtaining analysis sample. (A) Artistic drawing of the location of the liver in the human body. Source: http://www.northarundel.com/patiented/articles/liver_000421.htm; (B) Vial used today for sample fixation; (C) Schematic drawing of a viscerotome. Source: http://www.northarundel.com/patiented/articles/liver_000421.htm; http://profiles.nlm.nih.gov/VV/B/B/K/Q/(D) Picture of the viscerotomy section of the Yellow Fever Laboratory. Source: http://profiles.nlm.nih.gov/VV/B/B/F/M/ |
In the laboratory, the sample was cut in smaller fragments (cleavage), and one of the fragments was sent for processing, being also sent to slides for diagnosis purposes, and the other fragment was stored as technical spare sample.
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Cleavage: Image representing the cleavage technique in a liver sample. Technical Spare Sample: Picture of original vials of the Yellow Fever Collection with spare samples of the cases processed by the Yellow Fever Histopathology Laboratory. |
Before 1950 and before the invention of the automatic processor, tissue processing was a manual activity, and it was carried out by changing the histological sample in a binder solution to solutions with different alcohol grades, bleaching agents and liquid paraffin for tissue impregnation. After that, tissues were placed in paper molds. Blocks were trimmed to microtomy size. The process took several days, depending on the amount of work.
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Left image: steps of manual tissue processing. Right image: example of an automatic processor used nowadays in the conduction of the majority of the pathology activities. |
The last processing step was impregnation of the processed tissue in hard medium, with paraffin. Tissue is impregnated in the liquid form, forming a paraffin block with tissue inside, when solidified. This step is the inclusion step.
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Tissue inclusion procedure |
When the block is ready, Microtomiaque is conducted, a procedure for obtaining long tissue slices with a razor in a microtome. The microtome has undergone technological advancements to allow fine tissue sectioning (usually 5 micrometer) and semi-fine tissue sectioning (1 to 3 micrometers).
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Microtomy of tissues inside paraffin |
The histological preparation is colorless after microtomy, and it is necessary to make observation clearer. For this reason, staining substances are used to stain tissue, these substances are called stains. Finally, when tissues are stained, the sections are positioned in a way they are safe and preserved, and this step is called sealing (or assembly).
The tissue is ready for microscope analysis.
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Staining, sealing, microscopy and filing of histological sections.. |
