Several technics were employed for the application of the electron microscope to the study of the blood cells. First; lymphocytes, macrophages, and neutrophilic leukocytes of acute inflammatory lesions in man were prepared by imbedding formvar-covered screens in the corium of the forearm. Exudative cells which migrated to the undersurfaces of the specimen screens were quick-frozen and dehydrated in vacuo, according to a modified Altmann-Gersh technic. In some cases formvar-covered glass cover slips were substituted for the screens in the lesions, in which case an extra step was needed to transfer the preparations to specimen screens.

In a further technic, blood cells of lymph nodes, spleen and bone marrow of man were imprinted upon glass slides covered with formvar. These preparations were also frozen and dehydrated in vacuo. The films were then transferred to screens by a direct method in which the cells were retained intact.

Neutrophil granules, seen only as irregular, indistinct granules with the optical microscope, are actually round, oval, or rod-shaped structures measuring from 70 by 85 millimicrons up to as much as 465 by 650 millimicrons.

Immature blood cell precursors were found to possess fine chromatin patterns with distinct chromatin-parachromatin distinction and well-demarcated nucleoli.

At the higher magnifications made possible by electron microscopy, a new level of cytostructural organization appeared in the mononuclear phagocytes or macrophages. Individual fibrillar structural units can be made out in both the nuclear and cytoplasmic protoplasm which can be measured in Angstrom units. The nuclear protoplasmic interstices so formed are smaller than the corresponding cytoplasmic areas. The nuclear membrane is actually an anchoring structure for both nuclear and cytoplasmic fibrillar cross-pieces. Phagocytosis is depicted and the wall of the phagocytic vacuole is described. Finally, the observed cyto-structural features of the blood cells are discussed in the light of newer concepts of the finer structure of protoplasm.

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