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The Ultimate Guide to Tissue Processors
Need for Tissue Processing

Tissue processing is a crucial step in preparing histopathological samples. For microscopic examination, the tissue slice should ideally be 3–6 microns thick;however, the slice that can be achieved from fresh tissue is much thicker. Therefore, since fresh tissues from animals or humans are delicate and difficult to section, these require “processing” to remove water from cells and replace it with a solidification medium that can help cut thin sections for further analysis.

The steps of tissue processing include
  • tissue fixation for hardening and stabilizing tissues with minimal cell damage,

  • dehydration for removing water and fixative from the tissue specimens,

  • clearing for removing dehydrating solution,

  • infiltration for support medium permeation in the tissue, and

  • embedding for aligning tissue specimens in the support medium for solidification.

Revolutions in Tissue Processors

Initially, tissue processing was performed manually, which is a slow, laborious, and expensive procedure that exposes the technician to hazardous reagents. It also increases the chances of human error, ultimately leading to loss of precious tissue samples. Therefore, manual tissue processing was superseded by automated tissue processors around 20th century. Tissue processors can be broadly categorized as tissue transfer processors and fluid transfer processors.

Intissue transfer processors, also known as carousel tissue processors, the cassettes or capsules with tissue specimens are placed into perforated baskets, and the tissue is transferred to different reagents. This is achieved by arranging stationary reagent containers in a circular or line arrangement, and the basket revolves from one reagent dish to another for a set time period. The most common of these are the carousal or rotary type that can accommodate 9 to 10 reagents and 2 to 3 wax positions, thus offering a wide range of reagent choice and corresponding schedules.Their cassette capacity varies from 30 to 110, depending on the model, androtary motion or vertical oscillation of the tissue basket results in fluid agitation. The processors have pin, card-notched, or touch pad programs, and have a faster turn-around time for day or night processing. The recent versions have the advantage of an enclosed tissue basket within the integrated fume system with filters to absorb hazardous vapours that can arise during agitation or processing.

Towards the end of 20th century, fluid transfer processors, also known as floor standingtissue processors, were developed in which tissue specimens are placed in a retort, pumping in and draining out of reagents at a set time. These can accommodate 10 to 12 reagents and 3 to 4 wax positions. Their cassette capacity varies from 100 to 300, depending on the model, and tidal action results in agitation.Floor sStanding vacuum tissue processors have dramatically reduced processing times due to vacuum pressure cycles and heated reagents, which also improve the infiltration of dense tissue specimens. While carousel tissue processors face a major drawback of operator exposure to hazardous fumes, the floor standingtissue processor overcome this as the retorts are sealed and vapours are retained in the closed-loop system. These are programmed with microprocessors with a controlled screen display along with alert option and help settings for troubleshooting and system maintenance allowing easy operation.

Evaluating the Performance

Both carousel tissue processor and fully enclosed tissue processorrequire solvents, such as paraffin, xylene, alcohol, and formalin, as well as physical agents, such as agitation, convection heat, and vacuum/pressure. Due to increased time requirement, which is mostly >8 hours, the procedure is performed overnight, resulting in delayed sample preparation for microscopic analysis, further delaying the diagnosis report by a day. These instruments also share a drawback of safety issue during overnight process, thus requiring regular monitoring and the need for batch processing that adversely affects the workflow, so having a remote support to monitor/alert the user in case of any issue is a blessing. Fully automated floor standing tissue Processor also make it possible to run rapid cycle for processing small biopsies which can process tissues in just 3-4 hours.

Changing the World of Histopathology

Therefore, the advanced automated tissue processors have multi-fold applications in histopathology labs in research and hospitals. These offer the benefit of processing multiple samples at the same time, which results in decreased turnaround time, making it extremely valuable when a large number of patient samples are handled.The advanced versions also guard the samples against any damage that can occur due to unforeseen circumstances, such as power failure or improper handling of the operating system, thus solving the primary aim to avoid sample damage or loss. To improve applicability, some advanced processors are also equipped with operating software in diverse languages, with a solvent-resistant color touch-screen make it robust and long-lasting. These ensure complete data documentation and are convenient for subsequent data analysis and printing. The risk of exposure to harmful chemical reagents during sample processing is eliminated, providing a safer laboratory environment without compromising processing quality.

Which One to Prefer?
Advantages of floor standing tissue processor over carousel are as follows :
  • Fully enclosed, avoids exposure of operator to hazardous fumes

  • Low reagent level warning

  • Reagentmanagement system

  • Rapid cycle for biopsies can be run (3–4 hours processing time)

  • Formaldehyde can be pre-heated before send to SPC for faster fixation of unfixed tissues. No 14 hours outside fixation required as heated formalin can fix tissue in just about 2–3 hours.

  • Remote support/monitoring

  • Processing capacity of up to 300 cassettes in comparison to 100 maximum in carousel

  • Low running cost as reagent lifecycle is improved because of no evaporation to the environment

  • Can have 4 wax changes in comparison to only 2 in the carousel model

  • Some models can have two processing chambers, one of overnight process and one for small biopsies

Take-home message

Shifting to floor standing tissue processors is the need of the hourfor histopathology labs in hospitals and research facilities of all levels and capacities. Theseoffer the benefit of operator safety, low running cost, and simultaneous and error-free processing of tissue samples, thus significantly decreasing theturnaround time