An ultrastructural study on the multinucleation process of mouse alveolar macrophages induced by 1α,25-dihydroxyvitamin D3

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Abstract
The multinucleation process of isolated alveolar macrophages induced by 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] was examined using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). At the beginning of culture, most of the macrophages were spherical in shape. During incubation with 1.2 × 10−8 M 1α,25(OH)2D3, spreading macrophages appeared among the spherical macrophages, and they increased in number. Spreading macrophages extended many cytoplasmic processes toward adjacent macrophages, and interdigitations of these processes between those of neighboring cells were often seen. Two types of cell contact have been observed in the 1α,25(OH)2D3-treated cells. In some, cytoplasmic processes were put into the cytoplasm of the adjacent cells, where clathrinlike structures were observed at the inner membrane of the concave portion. In others, spreading macrophages occasionally came in contact with adjacent cells by a peripheral rim of their cytoplasm with gap junctions. Cytoplasmic continuity was rarely observed at the boundaries between the closely associated cells. The two types of cell contact were also found, though not frequently, in the untreated cells. These results indicate that 1α,25(OH)2D3 promotes multinucleation of alveolar macrophages through spreading forms with the formation of gap junctions and the coated membrane invagination.