Pathology Processing
last authored: June 2010, Kimberly Ordinelli
last reviewed:
Introduction
The tissue arrives in the lab in appropriate fixative accompanied by the requisition. Requisitions must be filled out correctly and match the label on the specimen. The lab assistant or technologist then accessions it to give it a unique identification number.
Tissues are initially grossed, usually by a Pathology Assistant (PA). Grossing consists of examination of the tissue and removing a sample from the area of interest.Size and description of the tissue is recorded, and tissue is placed it into a cassette. The cassette is then placed into the processor. Processing has many important steps involved that need to be followed in order: formalin, alcohols, xylene, and paraffin.
If processing overnight, tissues must be kept in formalin while it waits to be processed. Dehydration is the first step, acting to remove the water from the tissue to help prepare it for a nonaqueous solution which is used for infiltration. This is done with different concentrations of alcohols, the standard order as follows; 70%, 95%, 95%, 100%, 100%, 100%. This order can vary depending on lab protocol or tissue types.
Clearing reagents, such as xylene, have a high index of refraction which will leave the tissue transparent. It also removes the alcohol that was used for dehydration and primes tissue for the infiltration medium.
The most common medium for infiltration is paraffin - wax with small amount of plastic mixed in. Infiltration is used to keep the cellular structure of the tissue intact so you can obtain thin sections. If dehydration and clearing is not done properly, the tissue won’t be completely infiltrated. This will show up when you go to stain.
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