The morphological aspects of Breda virus serotype 2 replication in intestinal cells of gnotobiotic calves were investigated by electron microscopy. Ultrastructural findings suggest a morphogenetic pathway involving cytoplasmic vesicles, the Golgi apparatus and the cell nucleus. Virus uptake probably occurs via a receptor-mediated endocytosis-like mechanism. Endocytotic vesicles then carry virus to an as yet undetermined site of uncoating. Masses of tubules having the same diameters as Breda virion cores are found in nuclei, suggesting a role for the cell nucleus in replication of nucleocapsids. Similar tubules, as well as complete virions, were found in the Golgi region, the apparent site of virus assembly. Virus-containing Golgi vesicles then presumably move to cell surfaces where they fuse with apical and baso-lateral cell membranes to release virions in a way that permits more than one viral replicative cycle to occur without damage to host cell integrity. Virions are elongated with rounded ends and measure 42 × 100·5 nm. The morphogenesis and replication of Breda virus most closely resembles that of Berne virus of the proposed family Toroviridae.


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