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What are Microtomes?

Microtomes are machines that cut extremely thin sections from a sample for applications in histology or pathology. The sample is analyzed using transmitted electron microscopy (TEM) or light microscopy. They use special metal, glass, or diamond blades, depending on the type of specimen and the desired thickness. A microtome  consists of a blade holding unit with a blade holder and clamped blade, the object clamp, an advancing mechanism, and a mechanism for adjusting section thickness.

 

Example of tissue sample cut with laser microtome.

Example of tissue sample cut with laser microtome.

Image Credit: bioPhotonics

Microtome Specifications

When selecting microtomes, buyers should specify the blade material and type, as well as the application.

 

Blade Material

Microtomes have metal, glass, or diamond blades.

·         Metal blades are used to slice sections of plant or animal tissues for light microscopy histology. Typically, these blades or knives are made of steel.

·         Glass microtome blades are used in sample preparation activities for light microscopy and electron microscopy applications.

·         Diamond blades may be industrial-grade or gem-quality.

o   Industrial-grade blades are used to slice hard materials such as bone and teeth.

o   Gem-quality blades for microtomes are used mainly in electron microscopy applications.

 

Types of Microtomes

The GlobalSpec SpecSearch database provides information about these types of microtomes,

·         Saw microtomes are designed for cutting hard and brittle materials such as bone and teeth. They slice through the samples by pushing the sample across a recessed rotating saw. Their cut thickness is > 30µm.

·         Sled microtomes place the sample in a fixed holder or shuttle which moves back and forth across a knife or blade. Typically, the sled rests upon a linear bearing and permits angular adjustment. Often, sled microtomes are used with biological materials and samples that are embedded in paraffin. Their cut thickness is between1 and 60 µm

·         Rotary microtomes are used to cut thin sections of samples visible only with the aid of a light microscope. They are often operated manually but can be motorized. The sample is cut upon the rotary action of a hand wheel which advances the sample towards a blade. Rotary microtomes are used with hard materials, such as samples embedded in resins or paraffin. The thickness for manual operation ranges from 0.5.µm to 60µm

·         Vibrating microtomes use a vibrating blade which facilitates the sample cutting. This microtome uses less pressure than would be required with a stationary blade and is often used for difficult biological samples such as nervous tissues, brain and spinal cord. The stability of the instrument reduces the mechanical forces on the tissue sample to ensure the retention of high-quality viable cells.

·         Infrared laser microtomes are used to achieve contact-free slicing. To provide precise control, only small pulses of infrared (IR) energy are imparted into the sample. Laser microtomes offer several advantages including non-contact cutting, no sample preparation for native tissue, no decalcification of hard tissue, and a cut with precision of 1µm. They are generally too slow for routine pathology applications.

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Laser Microtome cutting a biological sample. Image Credit: bioPhotonic

 

·         Cryomicrotomes are rotary microtomes that slice samples (cryosectioning) that have been frozen in a liquid nitrogen chamber. Reducing the temperature inside this chamber increases the sample’s hardness, allowing the preparation of sections with a very specific thickness.

·         Ultramicrotomes for preparing samples with extremely thin sections are also available.  Typically, these very thin cuts are used with transmission electron microscopy (TEM), serial block-face scanning electron microscopy (SBFSEM), and light optical microscopy. 

 

Comparing Microtomes

This table compares microtomes by sample type, sample cut thickness, and application.

 

Type of Microtome

Type of Sample

Sample Cut Thickness

Application

Saw microtome

Hard and brittle material

> 30µm.

Samples such as bone and teeth

Sled microtome

Embedded samples

1 – 60µm

Samples sliced at an angle

Rotary microtome

Embedded samples

0.5 µm - 60 µm

Thin samples. Manual control

Vibrating microtome

Difficult/soft, Fresh/fixed samples

Fixed >10 µm

Fresh > 30 µm

Less pressure and sample disruption

Laser microtome

All samples

>1 µm

No sample contact and no sample prep

Cryomicrotomes

Frozen samples

 

Very specific thickness

Ultramicrotomes

 

TEM 40-100nm

SBFSEM 30-50nm

Extremely thin cuts for analysis with specialty microscopes


References/Resources

http://www.micro-tec.de/englisch/microtome/microtome.htm

http://www.photonics.com/Article.aspx?AID=38890

http://rotarymicrotomes.com/rotary-microtomes/an-introduction-to-rotary-microtome/

http://myneurolab.com/myneurolab/default.aspx