Paraffin Processing and Embedding the Tissue

​​technician at labAfter surgical pathology, your tissue, still inside the cassette, makes its way to the 11th floor of Toronto General Hospital. Almost immediately, the specimen is processed to create a permanent block of tissue embedded in paraffin. 

Chemically speaking, processing really means extracting all the water from the tissue to allow for the penetration of paraffin wax, in which the tissue is finally embedded. 

Embedding the tissue occurs immediately after paraffin processing.  When the processed cassettes are removed from their molten wax bath, the histotechnologist opens each cassette one by one and scans the block identifier with their activity log to ensure we have everything we should, as described by the pathologist's assistant. 

When the cassette is opened, they scan the tissues inside and transfer the pieces into a metal mould that contains molten wax. The specimens are organized in the mould so that the most representative facets of tissue are available for subsequent studies.

The mou​​ld and cassette component containing the unique identifier for that tissue is then transferred to a cold plate to allow the wax to harden. We now have a permanent, solid block of wax that tissues can be stored in for many years to come. These blocks are retained by the laboratory for any future studies required for that patient.


Tissue Sectioning

After embedding, the blocks are ready to be cut on a machine called a microtome. The technologist first scans the block under an infra-red reader. The scanner software is integrated with the LIS, so the 2-D barcode on the cassette will tell the LIS which block on a patient's case is being handled. It then automatically prints all the slide labels the technologist needs for sections cut from that block.

The technologist clamps the block into the microtome so that it is held firmly, and trims the excess wax to expose the tissue.

The individual section or a ribbon of sections is floated onto the surface of a warm water bath which helps to flatten the section and remove any wrinkles. The technologist then chooses the best quality section, separates it from others with forceps, and picks up the chosen section onto a glass slide used for microscopic analysis.

The slides are permanently labelled with the unique identifier for that block and slide, and placed in a rack to dry. Drying removes water from the section and helps the section adhere to the glass slide. Paraffin section slides are then ready for staining.

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