ULTRASTRUCTURAL CHARACTERIZATION OF ENDOGENOUS RETROVIRAL PARTICLES ISOLATED FROM NORMAL HUMAN PLACENTAS

Human placental endogenous retroviral (ERV) particle isolates were inv estigated by ultrastructural evaluation. Although retrovirus-like stru ctures in normal human placental tissue sections have been described, the precise nature of these particles has not previously been defined. By direct electron microscopic (EM) observation of placental ERV isol ates that display retroviral reverse transcriptase (RTase) activity an d a buoyant density consistent with type-C retroviruses (1.17 g/ml on sucrose), we have shown these to contain particles with characteristic retroviral ultrastructural features. Samples were stained with 1.0% p hosphotungstic acid (PTA), 0.5% uranyl acetate (UA), and Low-angle-sha dowed with platinum/palladium. Isolated placental ERV particles have a n apparent diameter of approximately 120 nm, are membrane-bound with a short surface hinge, and contain capsid particles (about 90 nm in dia meter) within an internal matrix structure. These observations support the view that human placental cells normally express ERV particles.