Frequency and antigenicity of type C retrovirus-like particles in human placentas.

Seventeen placentas from normal Japanese women, obtained from 15 full term gestations and at 2 earlier gestational periods, were observed by electron microscopy for the presence of type C retrovirus-like particles. Immature free and budding particles were found in 16 placentas including those obtained pre-term. Free virus-like particles were seen in the intercellular spaces and within the lysosomes of syncytiotrophoblasts of the chorionic villi. Forms budding from syncytiotrophoblasts were found in 3 cases. Coincident data with the electron microscopic observations were obtained by immunohistological methods. Specific positive staining with antisera against heterogeneic whole murine leukaemia virus were seen within the cytoplasm of trophoblastic cells both in and along the basal lamina.