A microtome (from Ancient Greek μικρός mikrós 'small' and τέμνειν témnein 'to cut') is a precision cutting instrument used to produce extremely thin slices of material, known as sections. These sections are typically in the micrometer range and are prepared from various specimens, most commonly biological tissues, for examination under a microscope. The ability to cut uniform, thin sections is crucial for microscopic analysis, allowing light or electron beams to pass through the sample, revealing its internal structure and cellular details.
Principles of Operation
At its core, a microtome operates by precisely advancing a specimen block towards a sharp cutting blade, or vice versa, to remove a slice of a predetermined thickness. The material to be sectioned is usually embedded in a supportive medium, such as paraffin wax, epoxy resin, or frozen in an optimal cutting temperature (OCT) compound, to provide rigidity and facilitate thin sectioning without distortion. The cutting edge is typically a highly polished steel blade, a glass knife, or a diamond knife, depending on the hardness of the material and the required section thickness. The thickness of the section is precisely controlled by a micrometer screw or a digital stepping motor that incrementally advances the specimen.Types of Microtomes
Different types of microtomes are designed for specific applications, material types, and required section thicknesses:-
Rotary Microtome: This is the most common type for routine histology. The specimen block, mounted on an arm, moves in a rotary motion up and down past a stationary knife. As the block moves up, the feed mechanism advances it by a set increment, and then on the downward stroke, a section is cut. Rotary microtomes are typically used for paraffin-embedded tissues, producing sections usually between 2 to 10 micrometers thick.
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Sliding Microtome: In this design, the knife slides horizontally across a stationary specimen block. Sliding microtomes are often preferred for larger or harder blocks, such as those embedded in celloidin or large paraffin blocks, and for brain sections. They can produce sections from 1 to 60 micrometers.
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Freezing Microtome / Cryostat: A freezing microtome, or more commonly a cryostat, is used to cut frozen tissue samples. A cryostat combines a rotary microtome within a refrigerated chamber, maintaining temperatures typically between -10°C and -30°C. Freezing tissue allows for rapid sectioning without embedding, essential for quick diagnostic procedures like intraoperative consultations in surgery, where results are needed within minutes. Sections are usually 5 to 10 micrometers thick.
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Vibrating Microtome (Vibratome): This type of microtome uses a vibrating blade to cut unfixed or lightly fixed tissue without embedding. The vibratory action allows for the preservation of cellular morphology and enzyme activity, making it valuable for electrophysiology, immunohistochemistry, and other applications where tissue integrity is paramount. Section thickness ranges from 30 to 500 micrometers.
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Ultramicrotome: Designed for electron microscopy, ultramicrotomes cut extremely thin sections, typically in the nanometer range (e.g., 50-100 nm). These instruments use very sharp glass knives or precisely manufactured diamond knives to section epoxy resin-embedded samples. The ultra-thin sections are then collected on grids for viewing under a transmission electron microscope (TEM).
Applications
Microtomes are indispensable tools across various scientific and medical fields:- Histology and Pathology: The primary application is in preparing tissue samples for microscopic diagnosis of diseases (e.g., cancer) and for studying tissue architecture.
- Biological Research: Used extensively in cell biology, neuroscience, and developmental biology to prepare samples for detailed structural analysis.
- Materials Science: In some cases, microtomes can be adapted to section non-biological materials, such as plastics, textiles, or composite materials, for microscopic examination of their internal structure.
- Forensics: Tissue samples from autopsies are sectioned to identify causes of death or disease.