The ultrastructure of macrophages in granulomatous colitis of Boxer dogs.

Thirteen cases of granulomatous colitis of Boxer dogs were studied by electron microscopy to define the contents of macrophages and to seek infectious agents. Macrophages were of three types. The most numerous were distended with residual bodies composed of membranes and parallel pairs of membranes, many arranged in circular patterns. A second common form had heterogeneous cytoplasm distended with phagosomes containing numerous small granular spheres of various sizes and electron densities. The least common were "young" macrophages containing phagocytic vacuoles. Rare macrophages in four of 13 dogs contained bacteria. Macrophages in five of the dogs contained abundant coccoid, coccobacillary of lobulated granular structures, 100-500 nm in diameter, resembling chlamydia. Transition from phagocytic particle to phagosome to residual body was apparent. Granulomatous colitis of Boxer dogs is probably caused by a lipid-rich, ribosome-rich, coccoid to coccobacillary organism that possesses a cell membrane and sometimes a nucleoid, and ranges from 100 to 500 nm in size.