A method--"fracture label"--is described for the cytochemical labeling of the membrane faces produced by freeze-fracture. Human erythrocytes embedded in a crosslinked matrix are frozen, fractured in liquid nitrogen, thawed, labeled, and cut into thin sections. Electron microscope observation of the fracture faces shows preferential partition of concanavalin A binding sites with the inner half of the membrane. This signifies that, during freeze-fracture, binding sites are dragged from the outer surface across the outer ("exoplasmic") half of the membrane and retained on the protoplasmic fracture face (face P). The fracture process results in exposure of new anionic sites on face P. Fracture-label can be applied to the cytochemical characterization of the cellular components exposed by freeze-fracture of isolated cells and tissues.
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