The ultrastructural localization of cell surface glycoconjugates: affinity cytochemistry via the avidin-biotin complex

The use of the high affinity avidin-biotin complex as an intermediary for the specific ultrastructural labeling of cell surface glycoconjugates is reviewed. The biotin molecule can be selectively implanted onto membrane-based saccharides by various chemical and enzymatic means or via prior attachment to an appropriate biologically-active binding protein (e.g., lectin, antibody, hormone, etc.). The distribution of the biotin-modified constituents can then be qualitatively and quantitatively evaluated under the electron microscope by avidin, conjugated to an appropriate marker (e.g., ferritin). The method has been demonstrated to circumvent some of the problems relating to ferritin-protein conjugation. In addition, the use of the avidin-biotin complex offers a unified and facilitated approach for the ultrastructural labelling of cell surfaces. Since the biotin molecule is foreign to the experimental system, the method is especially appropriate for double-labeling and kinetics studies. The procedure is applicable for analysis of labeled material in thin sections, freeze-etched replicas, shadow-casing or negatively stained samples by transmission electron microscopy. The method can also be modified for scanning electron microscopy. Due to the flexibility of this approach, we anticipate a rapid rise in the future use of the avidin-biotin complex as an ultrastructural probe of cell surfaces.