A new fixation procedure for preserving the ultrastructure of marine invertebrate tissues

SUMMARY Addition of 0·05% OsO4 to a conventional glutaraldehyde fixative for the first 10 min of fixation was found to improve greatly the preservation of ultrastructure in the eggs of Urechis caupo. Several workers have since confirmed this result in other marine invertebrate tissues. Specific protocols and techniques are given. We believe that the OsO4 rapidly renders the plasma membranes of cells freely permeable to glutaraldehyde, allowing faster penetration of this fixative. This method should be applicable to a wide variety of tissues that are difficult to fix.