Tissue Choppers

Introduction

A tissue chopper is a cutting/slicing instrument used to cut fresh living tissue in cube slices and arterial rings without the need for freezing g or embedding. The chopping mechanism produces smaller portions of the tissue for effective diffusion of metabolites. It is especially utilized in diagnostic laboratories to slice smaller and irregular biopsy specimens in regular sections for optimum observation. Specimen slices prepared with the tissue chopper are also useful in metabolic and electrophysiological studies. 

Typical modes of tissue preparation like blending, homogenizing, and grinding produce cell-free tissue specimens. However, with its specialized functions of blade speed control and slice thickness, the tissue chopper yields tissue sections with minimized damage to the cell structure and allows the diffusion of metabolites. It is also used to cut fragments that are impossible to be sliced using manual techniques.

Tissue choppers are equipped with a chopping instrument, a stainless steel table, a blade holder, a chopping arm, a chopping disc, and a mounting disc. Tissue choppers can be used with a petri dish in place of the typical plastic chopping disc. Tissue specimens are placed on a sterilized cutting table that moves from left to right, allowing the double-ended blade to cut swiftly across the length of tissue and yield uniform slices. 

Principle

The tissue chopper is an electrically powered appliance, and therefore the blade or cutting speed and slice thickness can be determined prior to operate the chopper. Adjustments to the blade speed, which is around 0-200 strokes per minute, are made by rotating a speed knob. The slice thickness can be calibrated on a micrometer head (1 micron – 25 micron). 

Once the chopper is set to the desired settings, the tissue specimen is placed on the stainless steel table that traverses from left to right, allowing the blade or chopping arm to drop and cut against the tissue placed on the moving cutting table. The table speed and the orientation are also adjusted as per requirement. For prism cuts, the table is oriented at 45 degrees, whereas for cube cuts, a 90 degrees orientation of the table is essential.

Protocol

Prepare the specimen and conserve it before beginning the slicing/cutting procedure. Thoroughly inspect the chopper and parts for any damages without connection to the power supply. Place the chopper on a firm, sturdy slab or table near a properly grounded power outlet. Each division on the thimble of the micrometer head signifies 1 micron and each division on the barrel is 25 microns. Adjust the slice thickness with the micrometer head and the blade force by turning the blade force knob-clockwise to increase the force and anticlockwise to decrease it. 

For positioning the double-sided blade, 1) secure three or more filter papers of the required diameter by two spring clips on the circular plastic disc of the chopper, 2) Rotate the blade force knob clockwise until the chopping arm drops, 3) Separate the blade clamping plate, adjust the blade on the top of the filter paper along its length and tighten the nuts with a spanner. 

Plugin the power lead of the chopper into the electric outlet and switch it on. In operation, the blade will cut the top filter paper. Once cut, switch off the main supply and discard the filter paper. Bring the table into starting position by pulling the table release knob. Clean the table and the blade with 70% ethanol, and then place the specimen on the table, just under the blade. 

Then wet the filter paper and the blade with a few drops of the Hibernate A preparation medium such that the tissue specimen sticks to the filter paper and not the blade. Switch on the plug to begin slicing/cutting. Position the table at 90 degrees for cubed cutting of the specimen. 

Once cut, gently transfer the tissue slices to a 100 mm culture dish containing chilled medium and leave them in for 5 minutes. Transfer the tissue slices to a new 100mm culture dish with a lesser amount of chilled medium for ease in tissue handling. Then follow the protocol for further tissue processing in accordance with the type of tissue meant to be isolated and observed. 

Upon completing the procedure, unplug the chopper and disassemble the blade holder, the cutting table, and the plastic disc and sterilize them according to the manufacturer’s instructions. Wipe the chopper with a dry and clean soft cloth. When removing the blade holder for sterilization, slide a paper sheet under the blade holder to prevent the falling of nuts and washers into the instrument. Apply a little paraffin oil to the underside of a sterilized cutting table before mounting it. 

Applications

Nikolas Skopow and his team investigated the viability of adipose tissue ex vivo using a human slice culture system model. Only limited success has been attained with methodologies conducted on adipose tissue despite its involvement in the severity of several metabolic and chronic inflammatory conditions. This dearth of solid research findings may be attributed to the inherent complexity of the adipose tissue and the limitations of the study models. 

In the human slice culture model, human subcutaneous adipose tissue samples were obtained and dissected into 5x10x10 mm pieces and then cut into 350, 500, and 750μm thick slices with a tissue chopper. The cut pieces were subsequently transferred to cell inserts of 0.4μm pore size and then to six wells containing 1 ml of cell media and incubated at 35 degrees Celsius in a humidified CO2 incubator. The cell media was changed first at the 24-hour mark and then after 48 hours. All individual slices were then observed under a microscope, analyzed, and compared with slices fixed at day 0. Additional methods of analysis included macroscopic analysis, histological analysis, and immunofluorescent analysis. The trials and results of the study concluded that the methods used during the study effectively maintained the viability of human-derived adipocytes and confirmed the current findings of rodent adipose tissue. 

Brewer and his co-researchers conducted a protocol for isolating rat-derived neural cells. The team obtained rat brains with intricate dissection techniques. After dissection, hippocampal parts of the brain were separated and placed onto a cooled cutting table of the tissue chopper. The long axes of the hippocampi were placed perpendicular to the razor blade. 0.5 mm thick slices made with the chopper were immediately wetted with a preservative reagent and transferred to a 50ml tube filled with the same reagent at 4 degrees Celsius. The neural cells were then isolated from the prepared slices with trituration and centrifugation techniques.

Strengths

Conventional slicing methods involved freezing and embedding the specimen in paraffin wax (hardening) before beginning the manual slicing procedure. Freehand cutting of the animal tissue with a razor does not yield regular bordered slices and also destroys cell structure. With the tissue chopper, the freshly sectioned specimen is directly placed on the table for cutting, and the blade swiftly cuts through the specimen and forms uniform slices. Also, the blade force and the thickness of the slices can be conveniently pre-determined using the force control knob and calibration of the micrometer head, respectively. 

The automaticity of the process and additional control provided by the specifications fairly reduce the incidence of human error and also preserves the functional viability of the tissue being cut for observation purposes. Tissue choppers are especially preferred over homogenizers and blenders due to the advanced blade speed and slice thickness controls. 

When turned on, the safety switch provided with the tissue chopper prevents the over-running of the machine. It causes an immediate release mechanism for the table to return to its original position. Additional safety mechanism includes a pre-set safety stop that prevents the penetration of the attached blade into the filter paper and damage to the stainless steel table impacted by the blade. 

Limitations

The blade fixed in the chopper must be sharp enough to smoothly cut through tissue without any hindrances and form slices of uniform contour and thickness. A blunt blade, if used, damages cell structure, cuts slices with serrated borders and makes the tissue specimen unfit for further processing. In addition, some tissue choppers come with leads of different colors that have to be plugged in carefully to ensure the safe running of the machinery. Operators must consult the manual for accurate instructions as the colors of the leads may not correspond to the terminals in the power outlet. 

During cutting, the blade does not have any surrounding barrier, and therefore there is always a possibility of damage to other parts of the appliance, such as the cutting table. Hence, it is the operator’s responsibility to prevent the blade from damaging other instruments of the appliance. Since the blade is knife sharp, the operator has to practice stringent caution and keep away while the machine is running.

Summary 

1. A tissue chopper is an electrically powered appliance used to slice small and delicate biopsy specimens and also tissues that may otherwise be impossible or difficult to cut.
2. It consists of various instruments such as the blade holder, the cutting table, and the chopping arm, all of which work in harmony to produce finely chopped tissue slices
3. The speed of the chopping arm and the required thickness are both pre-determined by the operator.
4. A double-ended razor is fit into the blade holder to cut through tissue placed on the cutting table. 

References

1. Schommer, J., Schrag, M., Nackenoff, A., Marwarha, G., & Ghribi, O. (2017). Method for organotypic tissue culture in the aged animal. Methods X, 4, 166-171. https://www.methods-x.com/article/S2215-0161(17)30013-4/fulltext
2. School. N, et al. (2020). Examination of human adipose tissue slice culture. Plus One. https://doi.org/10.1371/journal.pone.0233152
3. Brewer, G. J., & Torricelli, J. R. (2007). Isolation and culture of adult neurons and neurospheres. Nature protocols, 2(6), 1490-1498. https://www.nature.com/articles/nprot.2007.207
4. McIlwain H. (1961). Techniques in tissue metabolism. 5. Chopping and slicing tissue samples. The Biochemical journal, 78(1), 213–218. https://doi.org/10.1042/bj0780213
5.