Osmium binding to the surface coat of intestinal microvilli in the cat under various conditions

The appearance of the carbohydrate‐rich surface coat covering the the intestinal epithelium of the cat varies with different methods of fixation for electron microscopy. A fibrillar surface coat covers the external surfaces of the microvilli after fixation in glutaraldehyde and postfixation in phosphate‐buffered OsO 4 ; its density is enhanced by heavy metal staining. In tissue fixed in glutaraldehyde alone or followed by dehydration in acetone and postfixation in OsO 4 in carbon tetrachloride, a structureless zone less dense than Epon occurs where the surface coat is usually visualized; heavy metal staining does not impart density to the zone. The absence of structure and low density of the zone are not due to extraction of the surface coat by dehydrating agents or by carbon tetrachloride. The data suggest that osmium binds to the surface coat from phosphate buffer, but not from carbon tetrachloride. Also OsO 4 in phosphate buffer, but not OsO 4 in carbon tetrachloride, facilitates heavy metal staining of the surface coat. Acidic carbohydrates appear to be important for the differential binding of osmium observed in this study. The need for caution in interpreting low density areas of electron micrographs as extracted structures and high density areas as “osmiophilic” structures is discussed.