microtome microtome a microtome (from the greek mikros, meaning "small", and temnein, meaning "to cut") is a cutting tool used to produce extremely thin slices of material known as sections, with the process being termed microsectioning. important in science, microtomes are used in microscopy for the preparation of samples for observation under transmitted light or electron radiation. microtomes use steel, glass or diamond blades depending upon the specimen being sliced and the desired thickness of the sections being cut. steel blades are used to prepare histological sections of animal or plant tissues for light microscopy. glass knives are used to slice sections for light microscopy and to slice very thin sections for electron microscopy. industrial grade diamond knives are used to slice hard materials such as bone, teeth and tough plant matter for both light microscopy and for electron microscopy. gem-quality diamond knives are also used for slicing thin sections for electron microscopy. microtomy is a method for the preparation of thin sections for materials such as bones, minerals and teeth, and an alternative to electropolishing and ion milling. microtome sections can be made thin enough to section a human hair across its breadth, with section thickness between 50 nm and 100 μm. history a diagram of a microtome drawn by cummings in 1770. in the beginnings of light microscope development, sections from plants and animals were manually prepared using razor blades. it was found that to observe the structure of the specimen under observation it was important to make clean reproducible cuts on the order of 100 μm, through which light can be transmitted. this allowed for the observation of samples using light microscopes in a transmission mode. one of the first devices for the preparation of such cuts was invented in 1770 by george adams, jr. (1750–1795) and further developed by alexander cummings. the device was hand operated, and the sample held in a cylinder and sections created from the top of the sample using a hand crank. in 1835, andrew prichard developed a table based model which allowed for the vibration to be isolated by affixing the device to the table, separating the operator from the knife. occasionally, attribution for the invention of the microtome is given to the anatomist wilhelm his, sr. (1865), in his beschreibung eines mikrotoms (german for description of a microtome), wilhelm wrote: the apparatus has enabled a precision in work by which i can achieve sections that by hand i cannot possibly create. namely it has enabled the possibility of achieving unbroken sections of objects in the course of research. other sources further attribute the development to a czech physiologist jan evangelista purkyně. several sources describe the purkyne model as the first in practical use. the obscurities in the origins of the microtome are due to the fact that the first microtomes were simply cutting apparatuses, and the developmental phase of early devices is widely undocumented. at the end of the 1800s, the development of very thin and consistently thin samples by microtomy, together with the selective staining of important cell components or molecules allowed for the visualisation of microscope details. today, the majority of microtomes are a knife-block design with a changeable knife, a specimen holder and an advancement mechanism. in most devices the cutting of the sample begins by moving the sample over the knife, where the advancement mechanism automatically moves forward such that the next cut for a chosen thickness can be made. the section thickness is controlled by an adjustment mechanism, allowing for precise control. applications microtome (c. reichert, vienna, 1905–1915). the most common applications of microtomes are: traditional histology technique: tissues are fixed, dehydrated, cleared, and embedded in melted paraffin, which when cooled forms a solid block. the tissue is then cut in the microtome at thicknesses varying from 2 to 50 μm. from there the tissue can be mounted on a microscope slide, stained with appropriate aqueous dye(s) after removal of the paraffin, and examined using a light microscope. frozen section procedure: water-rich tissues are hardened by freezing and cut in the frozen state with a freezing microtome or microtome-cryostat; sections are stained and examined with a light microscope. this technique is much faster than traditional histology (5 minutes vs 16 hours) and is used in conjunction with medical procedures to achieve a quick diagnosis. cryosections can also be used in immunohistochemistry as freezing tissue stops degradation of tissue faster than using a fixative and does not alter or mask its chemical composition as much. electron microscopy technique: after embedding tissues in epoxy resin, a microtome equipped with a glass or gem grade diamond knife is used to cut very thin sections (typically 60 to 100 nanometer). sections are stained with an aqueous solution of an appropriate heavy metal salt and examined with a transmission electron microscope. this instrument is often called an ultramicrotome. the ultramicrotome is also used with its glass knife or an industrial grade diamond knife to cut survey sections prior to thin sectioning. these survey sections are generally 0.5 to 1 μm thick and are mounted on a glass slide and stained to locate areas of interest under a light microscope prior to thin sectioning for the tem. thin sectioning for the tem is often done with a gem quality diamond knife. complementing traditional tem techniques ultramicrotomes are increasingly found mounted inside an sem chamber so the surface of the block face can be imaged and then removed with the microtome to uncover the next surface for imaging. this technique is called serial block-face scanning electron microscopy (sbfsem). botanical microtomy technique: hard materials like wood, bone and leather require a sledge microtome. these microtomes have heavier blades and cannot cut as thin as a regular microtome. spectroscopy (especially ftir or infrared spectroscopy) technique: thin polymer sections are needed in order that the infra-red beam can penetrate the sample under examination. it is normal to cut samples to between 20 and 100 μm in thickness. for more detailed analysis of much smaller areas in a thin section, ftir microscopy can be used for sample inspection. a recent development is the laser microtome, which cuts the target specimen with a femtosecond laser instead of a mechanical knife. this method is contact-free and does not require sample preparation techniques. the laser microtome has the ability to slice almost every tissue in its native state. depending on the material being processed, slice thicknesses of 10 to 100 μm are feasible. sectioning intervals can be classified mainly into either: serial sectioning: obtaining a continuous ribbon of sections from a paraffin block and using all for slides. step sections: collected at specified depths in the block. types sledge a sled microtome a sledge microtome is a device where the sample is placed into a fixed holder (shuttle), which then moves backwards and forwards across a knife. modern sled microtomes have the sled placed upon a linear bearing, a design that allows the microtome to readily cut many coarse sections. by adjusting the angles between the sample and the microtome knife, the pressure applied to the sample during the cut can be reduced. typical applications for this design of microtome are of the preparation of large samples, such as those embedded in paraffin for biological preparations. typical cut thickness achievable on a sledge microtome is between 1 and 60 μm. rotary a rotary microtome of older construction this instrument is a common microtome design. this device operates with a staged rotary action such that the actual cutting is part of the rotary motion. in a rotary microtome, the knife is typically fixed in a vertical position. principle of sample movement for making a cut on a rotary microtome in the figure to the left, the principle of the cut is explained. through the motion of the sample holder, the sample is cut by the knife position 1 to position 2, at which point the fresh section remains on the knife. at the highest point of the rotary motion, the sample holder is advanced by the same thickness as the section that is to be made, allowing the next section to be made. the flywheel in many microtomes can be operated by hand. this has the advantage that a clean cut can be made, as the relatively large mass of the flywheel prevents the sample from being stopped during the sample cut. the flywheel in newer models is often integrated inside the microtome casing. the typical cut thickness for a rotary microtome is between 1 and 60 μm. for hard materials, such as a sample embedded in a synthetic resin, this design of microtome can allow good "semi-thin" sections with a thickness of as low as 0.5 μm. cryomicrotome