The Cross Striation Pattern of the Fibrin Fibril

Two different cross striation patterns can be seen under the electron microscope when fibrin fibrils are treated with heavy metal salts. The first shows two bands of different intensities per period (Type I), while the second has four bands per period (Type II). In this work, the factors involved in the formation of this particular cross striation pattern have been investigated. The origin of the Type I pattern is attributed to a reaction between uranyl acetate or phosphotungstic acid with the polar groups of fibrin (positive staining). Type II pattern arises from the embedding of excess heavy metal salts in the loosely structured portions of the fibrin fibril (negative staining). The sites of accumulation of the polar groups are not the areas of greatest mass density of the molecule.