Further observations on the morphogenesis of human rotavirus in MA 104 cells.

Human rotavirus "KUN" strain was cultivated in a fetal rhesus monkey kidney cell line, MA 104 cells. Four types of virus particles in cells infected with KUN strain were clearly identified: nucleoid cores, single-shelled particles, double-shelled particles, and membrane band, "enveloped" particles. "Enveloped" particles were found only in the thin sections of infected cells. When first visible, the virus precursors appeared at the ribosome free membrane of rough endoplasmic reticulum (RER), increasing in size while simultaneously being coated with nucleocapsid, inner shell. Single-shelled particles were also synthesized within bundles of filaments of viroplasm in the cytoplasma. During subsequent virus maturation two types of "budding" processes were observed. Double-shelled particles arising at the RER membrane entered the cisternae of the RER through an exocytosis-like process. In contrast, the "enveloped" particles developed in the cisternae by being completely enclosed with RER membrane, and later during cytolysis released the single-shelled particles. These "enveloped" virus particles appeared to be the result of inefficient virus maturation at the last stage of outer capsid formation.