Entry of Bombyx mori cypovirus 1 into midgut cells in vivo.

In vivo entry of Bombyx mori cypovirus 1 into the silkworm midgut was studied by electron microscopy of ultrathin sections of midguts from silkworm larvae that had been administered virus-contaminated leaves. In 3 h, virions were observed outside and inside midgut cells, including columnar cells, goblet cells and muscle cells. Virions were seen adhering to the plasma membrane of microvilli, embedding in the membrane and settling themselves intact inside the microvilli of the columnar cells. These results suggested that intact virions entered columnar cells by means of direct penetration through the cell membrane. In 12, 24 and 48 h, virogenic stromata and progeny virions were observed in columnar cells, but not in other midgut cells.