No abstract

Bei der mit Thiosemicarbazid umgesetzten Dialdehydstärke tritt ein gleichzeitiger Fixier‐ und Kontrastiereffekt erst bei der nachträglichen Behandlung mit Silbernitrat auf. Die Fixierung wird auf die durch freiwerdende Protonen begünstigte Bildung interchenarer Acetalbrücken zurückgeführt, die vornehmlich in den dichter gepackten und daher weniger oxydierten, nicht‐kristallinen Strukturbereichen der Stärke auftreten. Bei Behandlung der Ultradünnschnitte in OsO4 und Wasserdampf bei Temperaturen zwischen 50 – 60°C werden die Acetalbrücken reversibel gespalten, wobei die freigesetzten Aldehydgruppen spontan mit OsO4 reagieren. In derart behandelten Schnitten beobachtet man radial orientierte fibrilläre Strukturen. In den amorphen Bereichen hingegen wird die Feinstruktur der Amylose bereits bei der Perjodatoxydation zerstört, und die Abbauprodukte bilden feinste, durch Silber kontrastierte Präzipitate. Die kristalline Fraktion des Amylopektins kann ihrerseits durch Nachkontrastierung mit PWS (Phosphor‐Wolframsäure) elektronenmikroskopisch dargestellt werden. – Am Beispiel der im Verlauf der Teigverarbeitung in den Stärkekörnern auftretenden Strukturmodifikationen wird die Anwendung der Methode demonstriert.

The morphological appearance of cotton fibers scoured at 100 and 140°C then subjected to a treatment with cellulase, has been examined as a function of the enzymatic action time. The rate of enzymatic digestion varies according to the region of the secondary wall under consideration, and examina tion of the degraded fibers by optical and scanning electron microscopy clearly shows the bilateral structure of cotton. Four zones can be distinguished in the secondary wall, which differ in their accessibility to reagents. These differences can be attributed to local variations in the packing density of the fibrillar structure, caused by the mechanical forces which appear in the fibers during their drying. The effect of scouring at a higher temperature is to increase the rate of enzymatic degradation without affecting the relative and important differences in the resistance to biodegradation of the different regions of the secondary wall.

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 polymerization 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 oxidation, 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. _

Electron Microscope Contribution to the Study of Fine Structure of Wheat Starch. Electron microscope investigations of the fine structure of wheat starch granules were carried out using a combined fixation and staining technique. It was found that amylose is particularly concentrated in the nucleus. Uranophilic layers seem to indicate the presence of protein in the outermost surface regions of the starch granules, The radial orientation of the helicoidal unit chains of amylopectin could be confirmed. In both tangential and radial sectioning directions characteristic features of hexagonal structure are shown.