Electron microscopy of fibrinogen and fibrin.

It has been demonstrated through electron microscopy and x-ray diffraction that there are present, in certain protein fibrils, structural periodicities having dimensions up to several hundred angstrom units. Examples are collagen (l-3), paramyosin (4,5), the trichocysts of Paramecium (6, 7), and fibrin (8). The application of phosphotungstic acid (4) and other heavy metal compounds greatly enhances the visibility of the structures in electron micrographs and often makes structural variations apparent which are not perceptible in the unstained fibrils. It has not been established definitely whether staining involves a selective chemical reaction. However, since metal shadowing reveals in most cases that the fibril surface is corrugated with the high points corresponding to the highly staining region, it seems probable that stain deposits according to the local concentration of protein. The regions which stain more lightly, appearing as slight depressions on shadow-casting, probably represent portions which have shrunk during drying. Little, if anything, is known concerning the genesis of these macro periods or the reason for their occurrence in protein fibers. The investigation to be described was aimed at a clarification of these phenomena and is confined to fibrin and the precursor, fibrinogen.