Open tissue processor. Image courtesy of Wikimedia Commons.
An early automated version of manual processing, the “dip and dunk” processor automatically moved tissue cassettes to various reagent containers. Most models are unsafe without the use of a fume hood since the open reagent containers allow evaporation of chemicals, such as xylene. Alcohol also absorbed water from the air, making it difficult to keep alcohol concentrations close to 100% as needed for complete dehydration. They also lacked pressure, vacuum, and temperature control and therefore processing took much longer than today’s models.
Tissue cassettes remain in a closed chamber and reagents are drawn in from external containers and pumped back out. Since reagent containers are closed, including the main processing chamber, this is a safer method with little chemical evaporation. Closed processors, like the one shown in the lower image, have the ability to maintain vacuum and pressure, as well as control temperature for faster and more consistent processing.
These special laboratory grade processors are used to microwave tissues/reagents for faster processing. Some models require manual reagent changes between processing steps and careful temperature monitoring. Newer models are very similar to the standard closed-chamber processors with the addition of gentle tissue/reagent heating for rapid turnaround time. Tissue size must be carefully controlled for optimal microwave processing, mainly used for biopsies and routine tissue sections less than 3 mm in thickness.