Ultrastructural Characterization of Endogenous Retroviral Particles Isolated from Normal Human Placentas1

Abstract

Human placental endogenous retroviral (ERV) particle isolates were investigated by ultrastructural evaluation. Although retrovirus-like structures 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 isolates that display retroviral reverse transcriptase (RTase) activity and 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% phosphotungstic acid (PTA), 0.5% uranyl acetate (UA), and low-angle-shadowed with platinum/palladium. Isolated placental ERV particles have an apparent diameter of approximately 120 nm, are membrane-bound with a short surface fringe, and contain capsid particles (about 90 nm in diameter) within an internal matrix structure. These observations support the view that human placental cells normally express ERV particles.

Viral Particles of Endogenous Betaretroviruses Are Released in the Sheep Uterus and Infect the Conceptus Trophectoderm in a Transspecies Embryo Transfer Model

“…In fact, the presence of viral particles in mammalian placentae has been documented in many species, including humans, baboons, and mice (25,26,29,39). Although it has been always assumed that these particles derive from ERVs expressed in the placenta, it is also possible, as shown in this study, that these particles are released from the uterine endometrium and then enter into the mononuclear trophectoderm cells of the gestating conceptus.…”

Endogenous retroviruses and animal reproduction

“…The placenta is the major site of ERV expression with detection and isolation of retroviral particles from human placentas (Lyden et al, 1994), correlation of placental extracts and retroviral isolates with the presence of RT (Kato et al, 1987;Lyden et al, 1994) contributing to the identification of several endogenous families. This ERV expression-rich tissue has allowed the depiction of ERV regulation mechanisms for neighbouring genes.…”

Human Endogenous Retroviral Sequences: Possible Roles in Reproductive Physiopathology1

“…Since the mid-70s, several papers have reported the detection of ERV-related particles and/or antigens in the placenta, and especially in syncytiotrophoblast, of New and Old World nonhuman primates and humans [4]. HERV-related p30 (gag) protein and particles are found in the syncytiotrophoblast, close to the basal membrane [57,58]. The expression of HERV-related antigens changes along with trophoblast differentiation; only syncytial cells show a unique focal localization, mostly submembranous, in the cytoplasm, whereas mononuclear trophoblasts, like other tissues, show a diffuse reactivity [59,60].…”