An in vitro monocyte-macrophage colony-forming cell (M-CFC) has been detected in canine bone marrow (BM). The colonies derived from these progenitor cells were similar to murine-derived M-CFC (MacVittie and Porvaznik, 1978, J. Cell Physiol. 97:305--314) colonies, since they showed a singular macrophage line of differentiation, a lag of 14--16 days before initiating colony formation, and they survived significantly longer in culture in the absence of colony-stimulating factor (CSF) than granulocyte-macrophage colony-forming cells (GM-CFC). Endotoxin (Salmonella typhosa lipopolysaccharide W)-stimulated dog serum was used as the CSF (7% vol/vol). Canine-derived M-CFC progeny were identified as macrophages on the basis of morphology, phagocytosis, and the presence of Fc receptors for IgG. Gap junctions were observed only in canine BM, M-CFC-derived colonies using freeze-fracture and lanthanum tracer techniques. They were not observed in any GM-CFC-derived colonies. The number of gap junctions observed in freeze-fracture replicas of BM, M-CFC-derived colonies (21 colonies from three different dogs) showed a significantly positive correlation (Kendall's tau = 0.70, P less than 0.001) with the size of the colony fracture plane area. Gap junctions were observed displaying hexagonal lattices of 9.3 nm +/- 0.08 (SE) particles with a center-to-center spacing of 10.4 nm +/- 1.0 (SE) on membrane P-fracture faces. On membrane E-fracture faces, highly ordered arrays of pits with 8.7 nm +/- 0.12 (SE) center-to-center spacing were observed. Arrays of both particles and pits were also observed in fracture-face breakthroughs within a gap junction. Thus, gap junctions can form in vitro between the cells of macrophage progeny of a canine M-CFC under appropriate growth conditions. The significance of this observation is that there may be a structural basis for cell-to-cell collaboration between BM macrophages and other capable cells that either pass into the tissue for modification or develop there into mature cell forms.

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