Sections

Sections, microscopically thin tissue slices that make it possible to study tissues under a microscope without disrupting their structure and the interrelationship of their individual elements. For the preparation of sections, special apparatus—microtomes (see)—are used. To obtain the thinnest sections, paraffin or paraffin-celloidin embedding should be used. The size of sections may vary, depending on the size of the piece cut out. In most cases, they are made no larger than 18 mm; such sections are easier to obtain and more convenient to handle during subsequent staining. However, for certain special purposes, it is useful to obtain large sections, for example, through the kidney, uterus. Such large sections make it possible to study the structure of an entire organ or the topography of a process; they are made (by the Kristeller method) on special freezing microtomes with large stages. Instead of cover slips, mica sheets can be used here. Several factors influence the success of section preparation. First, it is necessary to have a certain and as uniform as possible density of the object, which is achieved by good embedding in paraffin or celloidin. When cutting on a freezing microtome, the piece should be frozen to the proper degree. Second, the sharpness of the microtome knife and its correct position are necessary. When cutting objects embedded in celloidin, the knife should be set at as acute an angle as possible to the specimen, that is, during cutting, the knife should pass through the piece with the entire length of its blade. When cutting paraffin-embedded specimens, as well as on a freezing microtome, the knife is usually set perpendicular to the piece. It is important to give the microtome knife the correct tilt—the lower edge of the knife's back should pass a few degrees above its blade (elevation angle). In microtomes not equipped with a device for correcting the elevation angle, this angle can be corrected by placing a metal or wooden plate under the knife where it is screwed on. The correct elevation angle is determined on trial sections. For the convenience of further processing and staining of sections, resp. to avoid damage to sections, sections are often mounted on slide or cover glasses. Paraffin sections should, as a rule, be mounted. Celloidin and frozen sections are mounted with special staining methods in mind or when it is necessary to remove celloidin from the sections. Mounting of paraffin sections is done as follows: the section from heated water, into which it comes from the microtome knife, is fished out onto a thoroughly degreased slide. Excess water is blotted with filter paper. The section is carefully pressed with a linen cloth folded several times and placed in an incubator at 37°. By the Japanese method, sections are also fixed by drying on slides, but the latter are first coated with a thin layer of glycerin-albumen. Well-beaten and filtered egg white is mixed with an equal amount of glycerin. The albumen mass is applied as thinly as possible to a well-cleaned slide and coagulated by heating to 70°. Slides prepared in this way can be stored until use. Celloidin and frozen sections are also mounted with glycerin-albumen, for which the sections are transferred from weak alcohol to slides prepared as described above. The sections are straightened and firmly pressed with filter paper. Then the slides are transferred to 96% alcohol, then to 70% alcohol and water. If it is necessary to remove celloidin, clove oil is poured onto the section, dried with filter paper. After the section becomes transparent, it is pressed with filter paper, and the slides are placed in 90% alcohol; then they are transferred to absolute alcohol and a mixture of absolute alcohol with ether until the celloidin dissolves, after which the slides are passed in reverse order through strong, weak alcohol and water. In cases where it is important to study the object sequentially throughout its entire thickness, a series of sections is prepared. Paraffin embedding is most convenient in this regard. The preparation of a series of paraffin sections is facilitated by the fact that with good paraffin embedding, each section adheres to the previous one, and thus a whole ribbon (the so-called taenia) of paraffin sections is obtained. Such sections are carefully removed from the knife and transferred to slides. Sections can also be fished out of water one by one and mounted on numbered slides by the methods described above. It is also convenient to proceed as follows: a little warm water is poured onto a large glass plate, onto which the sections are transferred directly from the microtome knife; they immediately straighten out, and with the help of a needle they can be arranged in the desired order. Excess water is allowed to drain off carefully. Many methods have been proposed for obtaining a series of celloidin preparations; the most common of them is the Weigert method. A thoroughly cleaned glass plate, preferably mirror glass, is coated with a thin layer of collodion (Coll. simplex). Sections are removed from the microtome knife sequentially with a strip of toilet paper (during the sectioning procedure, the sections on the paper should not dry, which is why the paper is placed on blotting paper moistened with weak alcohol). After a whole series of sequentially arranged sections has formed on the paper, a second, third, etc. ribbon is made in the same way. Then these strips of paper are arranged in a certain order on the prepared glass plate (sections downward), pressed against it, and then removed. The sections all transfer to the collodion. As soon as the sections begin to dry, the plate with sections is coated with a second layer of collodion. Thus, the sections are enclosed between two sheets of collodium and can be removed (under a layer of water!). Such a plate of sections can be stained in toto and subsequently cut into pieces. (See also Dehydration, dehydration in microscopic technique, and Degreasing.)—In recent years, the method of ashing sections has been introduced, which makes it possible to study the histological chemistry of tissues, in particular the composition of inorganic salts and metals in tissues, and also to apply the method of spectrophotography.