The Origin of Tissue Cassettes & Molds

by | Aug 1, 2019 | LabStore Highlights | 0 comments

Herbert Skip Brown, M. Div. HT(ASCP)
Lab Storage Systems, Inc.

Historically the Histology industry has seen a transition from the origin in tissue cassette and mold design that many would view today as rudimentary, even archaic, in its beginning. Histologists today oftentimes look at these beginnings and wonder how/why something as basic as cassettes and molds would have been made with such a tedious and cumbersome design. One must remember that design, manufacture, and even science was limited based on the then knowledge and technology of the time. There were no high-pressured moldings in cassette manufacture at that time. There were no precision press metal moldings for base molds. There was a limited understanding of fluidic flow of various solutions in tissues. All these things developed over a period of time due to advances in science, manufacture, and most important, the input and suggestions from Histology professionals. The ingenuity, manual dexterity, and commitment of those ancient warriors of Histology cannot be under-emphasized; because they were committed to using whatever tools, resources, or technology available at the time, to accomplish the task of preparing tissues to be microscopically examined and help provide accurate diagnoses for patient care.

Until the mid-1800’s, the primary emphasis after tissue processing was to encase the processed tissue in some form of containment item that would provide a barrier for the molten wax and allow it to congeal and solidify into a wax block. The block was then secured by various means to one of the early microtome instruments (which also was in its embryonic stage of development). As mentioned, the main emphasis was on containment of wax. In the 1950’s, noted pathologist and inventor Dr. James McCormick began to study the history of mold designs in Histology and created the graphic design images below; a period of design revolution from 1859 to 1959.

Much creativity was used in the design; from lead plates, to aluminum ice-tray type, to paper folded box molds, to plastic. All were uniquely designed to create a paraffin encasing that would allow you to hold the tissue in molten paraffin and allow it to harden into a block without spilling out. One of the primary challenges with these methods was specimen labeling. There was no system or method for labeling the specimen in the paraffin; and as you can see in the image (right), the paraffin block was melted onto a wooden or metal mount and this was secured into the microtome for sectioning. Once sectioning was completed the wax block was detached from the wooden mount and stored in a separate box or bag with labeling. To provide some form of label to the specimen, Histologists would take a thin strip of paper, write the patient number on it, dip it in wax to preserve the writing, and embed it in the wax block.

In the mid to late 1950’s Dr. James McCormick developed and patented a complete system of metal cast base molds and embedding rings, which allowed you to label each specimen block, mount onto a microtome, and store the sectioned block uniformly with a permanent label. This was a tremendous advancement in risk management because it significantly reduced the chance of lost label (strip), or ‘no’ labeling. This system was adopted by Lab-Tek Plastics Company and was introduced in the 1959 April issue of The American Journal of Pathology as the Tissue-Tek 1000 System. Dr. McCormick continued to develop and patent new innovations for processing, embedding, and microtomy, ultimately leading him to the universal design used today of the rectangular shaped standard tissue cassette.

The evolution of the cassette is no where near as creative and diverse as the evolution of molds, but it is important to note the milestone advances in science and manufacture that continued to promote fluid exchange within the tissue. Tissue processing was once done as simply as wrapping the tissue in sterile gauze and transferring it from solution to solution. This proved to be problematic when you had multiple or fragmented tissue parts. With this in mind, it was understood that some form of ‘containment device’ needed to be developed with the primary focus on holding the tissue but still allowing the process of fluid exchange. This meant that there had to be some degree of porosity in the device. Science had not caught up with manufacture and the mechanism for fluid exchange was simple holes drilled into the device. The evolution of this is seen below, as the transition in cassette shape, and the science of fluid exchange happened in congruity with each other. Note the transition from the clunky metal precursor to the cassette on the far left of the panel, with its drilled holes in the metal; to the plastic form of the same design but with more holes and a writing surface; to the universal design developed by Dr. McCormick with multi-channeled flow through vents on the bottom and lid of the cassette, and a writing surface on the front of the cassette for permanent labeling.

The science of infiltration of tissue, and the technology of manufacture had now merged to create a system which included labeling, processing, embedding, microtomy, and storage in one paraffin block. This system still reigns today as the standard in routine tissue preparation in Histology.

Advances in tissue cassette and embedding mold design continue today, as we have seen the development of fluidic flow cassettes with lateral vents on the cassette, screens, mesh, or intricate molding presses for biopsy cassettes, base mold designs that reduce or eliminate the incidence of scraping blocks, disposable base molds, and a plethora of other designs. Almost all of these design improvements have come from the expressed need of the user, the Histotechnologist. It is therefore important that we as health care professionals continue to voice our needs and suggestions to commercial vendors and manufacturers. It is our input that is at the root of so many patents in the industry, and it is our passion and commitment to improve our ability to serve the patient that will keep development and manufacturing focused on diagnostic excellence in the tools we use.

Lab Storage Systems is honored to give you this historical perspective on the products you utilize every day and has a complete line of cassettes, base molds, and ancillary items for routine and research applications. Contact our Customer Relations Manager for information on orders, or our Technical Marketing Specialist for technical assistance on product selection and use.

References:

1.Brown, H. Skip, ‘Small Specimen Management-In a Large Volume World’, Workshop in small specimen management, Lab Management Consultants, 2008.

2.McCormick, J.B., “From Hunks to Snippets-Part 1 – Embedding for Microtomy”,Workshop in historical innovations in embedding, Science Heritage, LTD, 2007.

3.McCormick, J.B., ‘Graphics images in embedding mold design: A Historical Perspective’, Science Heritage, LTD, 2007.

4.Lab Storage Systems, Inc., ‘Products in Embedding – Embedding Rings’, www.labstore.com.

5.The American Journal of Pathology, April edition 1959, Tissue-Tek 1000, Lab Tek Plastics Co.

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