Microtome

MIROTOME (from Greek mikros small and temno to cut), an apparatus for producing sections suitable for investigation under a microscope. The general principle of a microtome is that the object is raised in a vertical direction by a specific small amount expressed in microns, either directly by a micrometric screw or via an inclined plane; in the horizontal plane, a knife moves, being either held directly by hand (in the simplest models) or clamped in a special holder. For cutting on a microtome, the object must certainly be made as dense as possible and embedded in a homogeneous medium, e.g., paraffin or celloidin; the specimen can also be frozen using sprayed ether or liquid CO₂. Depending on the embedding medium, as well as the properties of the investigated specimen, the microtome makes it possible to obtain sections with a thickness of down to 1 µ (which is unattainable when cutting directly with a hand razor), as well as to obtain a series of sections of identical thickness without gaps. Types of microtomes. The hand microtome consists of a thick-walled metal cylinder with a well-polished disk on the upper side, across which an ordinary razor or special knife is guided by hand. Inside the cylinder is a fine screw thread; a micrometric screw operates within it, with a head at the bottom bearing divisions indicating how much the specimen has advanced within the cylinder sleeve. At the upper end of the screw is a platform upon which the object to be cut is mounted. The instrument is cheap and suitable only for coarse sections. -The student microtome is screwed to a table and consists of a sturdy metal frame in which a micrometric screw is mounted with an indicator of its travel and a platform for mounting the object or a clamp for a wooden or stabilit block onto which the specimen is glued. In the upper section of the frame is a polished arc along which the knife holder, equipped with a clamp, travels. The advancement of the specimen occurs either by hand or automatically upon each backward movement of the knife, for which purpose the screw is provided with a gear connected to the knife holder and equipped with a regulator that makes it possible to advance the specimen upward by a certain number of microns. Such a microtome makes it possible to obtain fairly thin sections. -A similar microtome is used for producing sections of objects frozen with liquid CO₂ ("freezing microtomes" by Jung, Reichert; Fig. 1). For this purpose, a sprayer for liquid CO₂ is attached to the object platform, supplied via a flexible metal tube from a steel cylinder, which is usually equipped with a reduction valve. The sprayer itself has a valve allowing the required jet of liquid CO₂ to be released or completely shut off to stop its flow. Sections from frozen objects have recently become very widespread, and for some methods of processing histological material they are completely irreplaceable, because they

make it possible to

process fresh,

material

with various reagents.

Furthermore, the "freezing microtome" eliminates troublesome and prolonged embedding, making it possible to quickly obtain the necessary preparation for patho-histological examination (Fig. 2).-Sliding microtome (Schanze and other firms). On a heavy cast-iron base, a vertical thick plate is mounted, to the left side of which another identical plate is attached at an acute angle, thus forming a receptacle for a wedge-shaped massive block upon which the microtome knife is mounted by means of a screw and special holder. On this wedge-shaped block and on

1. Freezing microtome "Sarto-..." — cylinder with carbon dioxide; 2 — metal tube leading to the sprayer (3) with the stage for the frozen object; 4 — knife; 5 — sprayer valve.

the sides of the angle formed by the aforementioned plates, polished strips are ground that coincide with each other and allow the knife carriage to slide smoothly in these ways. On the left side of the base... Figure 2. Sliding microtome: 1 — knife; 2 — object of investigation; 3 — one of the polished strips of the guide ways; 4 — dish for waste alcohol used to flood the knife; 5 — handle for coarse lowering and raising of the object; 6 — graduated disk. microtome there is a nut in which the micrometric screw operates. It stands vertically and has a disk on whose upper surface a series of divisions is applied; the value of the latter is engraved on it or on a special rod along which a spring-loaded clicker moves, counting the number of teeth by which the micrometric screw disk has been turned. Each tooth corresponds to a specific number of µ, so that the rise of the screw head can be set immediately to the desired number of microns. The head of the micrometric screw abuts against the object clamp, which moves up and down in a groove. This clamp can be adjusted to the desired position relative to the horizontal plane by means of screws securing the axes. A handle is attached to the holder, allowing it to be raised or lowered by an arbitrary amount without using the screw during the initial setup of the object and selection of the cutting plane. The microtome is very sturdy, stable, and very convenient for sections down to 5 µ in thickness. Thinner sections are already difficult to obtain on it. Jung microtome with an inclined plane (Fig. 3). Here, on the vertical plate of the microtome on the left, a second plate is mounted, also forming an acute angle, but positioned such that it descends at a slight angle toward the operator. It has polished strips upon which the wedge-shaped block with the object holder slides. In other models (e.g., Reichert), instead of a plate on the left side of the microtome, there is an inclined groove in which the object holder moves. Closer to the operator is the micrometric screw, resting with its pointed end against the object holder or a rod connected to it. The combination of an inclined plane and a micrometric screw makes it possible to raise the object by 1 µ and even fractions thereof. For convenience and continuity of operation, the screw can rotate on a horizontal axis and operate with one end or the other. In newer models, the screw nut is made in the form of pliers, which makes it possible to move the screw during setup without rotation. Connected to the screw is a circle with divisions in the form of small teeth, and the elevation height per 1 tooth is engraved. In larger models of this type, there is a device for automatically turning the screw by a specific number of teeth in conjunction with the backward movement of the knife. This device is most conveniently arranged in Reichert models, where the knife carriage abuts against an adjustable plate mounted at the front end of the guide ways. Pressure on it is transmitted by a rod to the screw, and the plate is returned to its position by a spring. The microtome with an inclined plane is very convenient and makes it possible to obtain sections down to 1 µ and even thinner (Apathy). The object holder is equipped with a clamp for the specimen, which can be well oriented when selecting the cutting plane using screws, gearings, or a ball joint (which is inconvenient). The brain microtome is designed to obtain large sections of the brain as well as other organs. Here, obtaining a very thin section is not required, but rather a section of large area. Such a microtome is usually constructed with a vertical screw and a very large platform for mounting the object. Since sections are most often prepared from material

Figure 3. Jung microtome with an inclined plane: 1 — alcohol reservoir; 2 — knife; 3 — object of investigation; 4 — inclined plate; 5 — mechanism transmitting the setting to the object holder.

embedded in celloidin, a flat reservoir for alcohol (or water) is arranged around the stage, and all cutting takes place in liquid, which facilitates the preservation of the integrity of the sections. Corresponding to the large size of the sections, knives for such a microtome are also used with a large cutting edge. - For cutting large objects, both frozen and embedded in celloidin or paraffin, the Jung firm recently released a new microtome called Tetram-der. This microtome consists of a heavy quadrangular metal frame on legs. Between the upper parallel bars

Figure 4. Microtome with a fixedly mounted knife (1) and a moving object (2).

To the frame of the microtome, to which the knife is immovably attached, there is an attachment for securing the object to be cut (a holder or platform); this part is brought into motion toward the knife by means of a special lever, and the object is automatically raised by a micrometer screw to a certain number of microns. When cutting objects embedded in celloidin, the microtome knife must be moistened with 70% alcohol. To avoid excessive alcohol consumption and for even application to the knife, a dropping apparatus is very convenient. It consists of a closed reservoir on a stand. A long tube with a tap delivers alcohol drop by drop from the apparatus, the rate of which can be regulated by the tap. In the models described, the object remains stationary while the knife moves. There is another type where the knife is stationary and the object moves, being mounted on a table that is positioned either horizontally, as in the Leitz model, or vertically, as in the Minot model, which is driven by a wheel similar to that of a sewing machine (Fig. 4). These models offer no particular advantages. On the same principle as the microtome, an apparatus for making thick sections for macroscopic examination (macro-tome) is constructed. For obtaining preliminary sections from fresh organs, a double knife can be used. It consists of small razor blades with handles fastened together by a screw. At the rear end of the handles, another screw moves a thin wedge, allowing the distance between the blades to be set, which are then secured in a fixed position parallel to each other by the aforementioned screw.