Contribution to the Electron-Microscope Study of the Fine Structure of Cellulose Fibers by Means of Ultramicrotomy

Some possibilities of using ultramicrotomy in an electron-microscope study of cellulosic fibers are considered. Interest is focused on the importance of preparative techniques, especially "fixation" and "inclusion." The well-known fact of expansion of the secondary wall (separation into lamellas of various thicknesses) is considered to be the consequence of polymerization of the methacrylate within the accessible regions of the swollen fibers after inclusion. Fixation, i.e., cross-linking, is considered to occur either in formaldehyde treatment or in periodic acid oxidation; in such cases, no poly merization takes place inside the fibers and, accordingly, no burstings are observed on thin sections. In the case of alkali treatments of sufhcient concentration, homogeneous swelling is observed and no fixation is necessary to prevent the sections from swelling when they come in contact with water in the ultramicrotome bowl. Some histochemical techniques, designed to enhance contrast in the electron image and reveal fine structure, are discussed. Silver ions can be used after periodic acid oxida tion, followed by thiosemicarbazide blocking; uranyl and lead give good results, especially when applied to fibers rich in free carboxylic groups; and caesium can be introduced through an exchange reaction by using alcoholates so that swollen structures can, thus, be studied.