Electron microscopy of mouse peritoneal macrophages infected with the Kümm stock of Cowdria
ruminantium suggests that in the final stage of intracellular growth, a mosaic of organisms develops
from an amorphous matrix of varying electron density by a process in which double unit membranes
portion off the Cowdria particles. This stage is preceded by inclusions consisting of a network of aggregated
electron dense granules and these in turn by homogeneous dense bodies. The study failed
to show how these dense bodies develop from internalized Cowdria particles introduced in the infective
inoculum. The replication of the heartwater agent in macrophages differs from that in vascular
endothelial cells in two important respects. First, at no stage during the course of development in
macrophages is binary fission in evidence and second, in the absence of a limiting membrane the
inclusions and colonies of organisms throughout the cycle of development in macrophages are in
intimate contact with the host cell cytoplasm.
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