Observations on transmembrane structures of surface immunoglobulin in the plasma membrane of B lymphocytes.

We have investigated the possible role of intramembraneous particles as revealed by freeze-fracture electron microscopy in the plasma membrane of B lymphocytes from rabbits and mice as reflections of transmembrane structures of surface immunoglobulin receptor molecules. This was achieved by aggregation of the surface receptors using fluorochrome-conjugated antibodies, fixation and freezing of the cells in 35% glycerol. This procedure resulted in replicas of lymphocytes with well-preserved morphology (no ice-crystals), enabling the study of both protoplasmic and external fracture face in combination with surface receptor markers. It appeared that very small intramembraneous particles (3-6 nm diameter) were selectively clustered under patches of surface receptor label. This phenomenon was found on the external fracture face exclusively and not on the protoplasmic fracture face. 'Classical' intramembraneous particles (6-12 nm diameter) were not involved. We suggest that these small, clustered particles should be interpreted as transmembrane structures of surface immunoglobulin molecules.