Ultrastructural changes in host cellular organelles in the course of the chlamydial developmental cycle

Different stages in the chlamydial developmental cycle were correlated with cytopathic changes in host cellular organelles by ultrastructural analysis of infected cultured cells and cells of the intestinal mucosa of newborn calves. Four distinct morphologic forms of chlamydial development can be distinguished: elementary bodies, dispersing forms, reticulate bodies and condensing forms which proceed to form elementary bodies. From the stage of elementary body uptake through formation and multiplication of reticulate bodies, the host cellular changes consisted mainly of displacement of cellular organelles by the enlarging chlamydial inclusions in the cytoplasm of infected cells. Beginning with the formation of condensing forms and elementary bodies from 20 to 30 hours after infection, cellular organelles became altered and progressively damaged. The damage was initially degenerative and then necrotic. It consisted of loss of free ribosomes and polysomes, dilation and vesiculation of the endoplasmic reticulum network and vesiculation and loss of microvilli. The specific granules of enterochromaffin cells and the mucous droplets of goblet cells were reduced in number. The mitochondria and nuclei were affected last. The mitochondria became swollen and their cristae became fragmented. The nuclei of infected cells lost their normal chromatin pattern and proceeded to pyknosis and karyolysis. Rupture and lysis of cytoplasmic and inclusion membranes liberated the different chlamydial developmental forms. After release the elementary bodies remained ultrastructurally intact.