Ultrastructure of human basophil leukocytes studied after spray freezing and freeze-substitution

Normal human blood cells were prepared for transmission electron microscopy by spray freezing, freeze-substitution and embedment in Araldite. Spray freezing was found to damage many cells. Nevertheless, the method developed preserved some leukocytes well and spray frozen, freeze-substituted human basophils showed interesting differences from conventionally prepared cells. Surface vesicles and tubulovesicular structures, extragranular myelin figures, and membranous projections on putative thin membrane-bounded granules were all absent. Preservation of the basophilic granules was clearly improved and their membranes were usually intact. The bulk content of these granules did not look particulate, but showed a very fine texture. Intragranular myelin figure material was well preserved and often enclosed striking lucent areas. Charcot-Leyden crystals were clearly seen in basophils from some donors, always in well preserved granules. A fourth type of material, not seen in conventionally prepared human basophils, was also often observed in the granules. Large lucent vacuoles could be regularly distinguished from basophilic granules. The granules with partly concave outlines, seen in conventionally prepared human basophils to be associated with ill-defined lucent areas and irregularity or projections of the membrane, were absent. In freeze-substituted cells, images suggesting fusion of basophilic granules with each other were seen quite frequently. It is concluded that, while the techniques needed to be improved, rapid freezing and freeze-substitution afford a better understanding of the ultrastructure of human basophil leukocytes.