Formation of giant cells in vivo.

In vivo, the development of multinucleate giant cells in granulomas always proceeds in accordance with a given mode (simultaneous phagocytosis--fusion--unordered multinucleate giant cells--ordered multinucleate giant cells). This mode is both quantitatively and qualitatively modifiable, showing a dependence on the quantity, shape, size, and digestibility of the phagocytic material: on uncoated coverslips implanted i.p. in rats, only a few small syncytia develop, bit by bit, on coverslips coated with phagocytotic material, on the other hand, numerous very large, always initially unordered, multinucleate syncytia showing simultaneous aggregation around large phagocytic objects are seen. These can develop into ordered giant cells of the Langhans type, by re-distribution of the cytoplasmic contents, but only when most of the phagocytosed material has been digested. Actively phagocytosing mononuclear macrophages, which coalesce or have already fused to form syncytia frequently develop various types of junctions that probably have the same function in granulomas as in epithelia. Pentalaminar structures in multinucleate giant cells are frequently observed in close conjunction with residual phagocytic material and obviously represent outer cell-membranes that have coalesced during fusion and been incorporated into the cytoplasm. The clear zones surrounding such areas probably represent an element of the cytoskeleton.