The use of osmium tetroxide-potassium ferrocyanide as an extracellular tracer in electron microscopy

Secondary treatment of glutaraldehyde fixed liver samples with long term stored solutions of osmium tetroxide and potassium ferrocyanide (Os-Fe) results in tracing of the extracellular spaces of the tissue with an electron opaque substance. This substance does not diffuse across cell membranes, its formation probably being related to the progressive reduction of the OsO4 molecule by potassium ferrocyanide. The application of the present method in electron microscopy may be useful in overcoming the artifacts often induced by other tracing techniques such as vascular perfusion with peroxidase or immersion in lanthanum. Although the period of storage of the Os-Fe solution is a clear disadvantage of the method, it seems plausible to anticipate that further studies on the chemical interaction between osmium tetroxide and potassium ferrocyanide will provide us with a Os-Fe mixture having an immediate tracer effect.